Add 4-bit quantization to DiskBBQ next

[tteofili]

This adds symmetric 4 bit support to DIskBBQ indices.

[tteofili]

this gives us a bump in recall over 1 and 2 bits, however it's currently much slower.
1-bit

index_name       index_type  num_docs  doc_add_time(ms)  total_index_time(ms)  force_merge_time(ms)  num_segments
---------------  ----------  --------  ----------------  --------------------  --------------------  ------------  
wiki1024en.docs         ivf    499000               836                 25145                     0             8

index_name       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count      QPS  recall    visited  filter_selectivity
---------------  ----------  -------------------  -----------  ----------------  -------------  -------  ------  ---------  ------------------  
wiki1024en.docs         ivf                 1.00         0.35              0.87           2.49  2857.14    0.74   13043.17                4.00
wiki1024en.docs         ivf                 5.00         0.44              1.43           3.25  2272.73    0.78   52763.14                4.00
wiki1024en.docs         ivf                10.00         1.01              3.43           3.40   990.10    0.79  102704.67                4.00
wiki1024en.docs         ivf                30.00         2.00              6.98           3.49   500.00    0.79  302511.11                4.00
wiki1024en.docs         ivf                50.00         2.71              9.43           3.48   369.00    0.79  501910.95                4.00
wiki1024en.docs         ivf                70.00         3.75             13.02           3.47   266.67    0.79  701296.15                4.00
wiki1024en.docs         ivf               100.00         5.22             18.33           3.51   191.57    0.79  998000.00                4.00

2-bits

index_name       index_type  num_docs  doc_add_time(ms)  total_index_time(ms)  force_merge_time(ms)  num_segments
---------------  ----------  --------  ----------------  --------------------  --------------------  ------------  
wiki1024en.docs         ivf    499000              1153                 26075                     0             8

index_name       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count      QPS  recall    visited  filter_selectivity
---------------  ----------  -------------------  -----------  ----------------  -------------  -------  ------  ---------  ------------------  
wiki1024en.docs         ivf                 1.00         0.99              3.28           3.31  1010.10    0.81   12898.42                1.00
wiki1024en.docs         ivf                 5.00         4.28             11.63           2.72   233.64    0.87   52753.10                1.00
wiki1024en.docs         ivf                10.00         8.24             21.96           2.67   121.36    0.88  102670.69                1.00
wiki1024en.docs         ivf                30.00        23.02             60.50           2.63    43.44    0.88  302414.15                1.00
wiki1024en.docs         ivf                50.00        37.37             96.84           2.59    26.76    0.88  502034.51                1.00
wiki1024en.docs         ivf                70.00        52.31            135.03           2.58    19.12    0.88  701306.98                1.00
wiki1024en.docs         ivf               100.00        76.94            198.21           2.58    13.00    0.88  997999.00                1.00

4-bits

index_name       index_type  num_docs  doc_add_time(ms)  total_index_time(ms)  force_merge_time(ms)  num_segments
---------------  ----------  --------  ----------------  --------------------  --------------------  ------------  
wiki1024en.docs         ivf    499000              1120                 26909                     0             8

index_name       index_type  visit_percentage(%)  latency(ms)  net_cpu_time(ms)  avg_cpu_count     QPS  recall    visited  filter_selectivity
---------------  ----------  -------------------  -----------  ----------------  -------------  ------  ------  ---------  ------------------  
wiki1024en.docs         ivf                 1.00         2.39              7.46           3.12  418.41    0.87   13259.86                1.00
wiki1024en.docs         ivf                 5.00         7.93             22.14           2.79  126.10    0.93   52943.50                1.00
wiki1024en.docs         ivf                10.00        15.46             42.11           2.72   64.68    0.94  102636.98                1.00
wiki1024en.docs         ivf                30.00        45.84            122.04           2.66   21.82    0.95  302375.00                1.00
wiki1024en.docs         ivf                50.00        76.42            201.22           2.63   13.09    0.95  501783.49                1.00
wiki1024en.docs         ivf                70.00       107.32            280.86           2.62    9.32    0.95  701135.30                1.00
wiki1024en.docs         ivf               100.00       164.37            418.40           2.55    6.08    0.95  998000.00                1.00

the slowness is partially expected because of the missing stuff in native computations (which would make sense to implement in a separate PR).
from this very initial tests, 2-bits is 2x slower than 1-bit and this one is 2x slower than 2-bits.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[john-wagster]

lgtm