Support index.length for MySQL 8.0.0-dmr

MySQL 8.0.0-dmr `SUB_PART` column of `information_schema.statistics`
changed to varbinary(12), which is bigint(3) in MySQL 5.6.

Addresses rails#26774

Author: yahonda

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -388,7 +388,7 @@ def indexes(table_name, name = nil) #:nodoc:
             end
 
             indexes.last.columns << row[:Column_name]
-            indexes.last.lengths.merge!(row[:Column_name] => row[:Sub_part]) if row[:Sub_part]
+            indexes.last.lengths.merge!(row[:Column_name] => row[:Sub_part].to_i) if row[:Sub_part]
           end
         end