feat: add workspace-proxy-url flag to scaletest workspace-traffic

[f0ssel]

This allows the command to target a workspace proxy when appropriate.

Part of coder/internal#149

So far I've verified the correct url being used with logs:

➜  coder git:(f0ssel/workspace-traffic-proxy) ✗ go run ./cmd/coder/main.go exp scaletest workspace-traffic --job-timeout=60s --workspace-proxy-url="https://paris.fly.dev.coder.com"
Running load test...

web url: https://paris.fly.dev.coder.com

...

➜  coder git:(f0ssel/workspace-traffic-proxy) ✗ go run ./cmd/coder/main.go exp scaletest workspace-traffic --job-timeout=60s --workspace-proxy-url="https://paris.fly.dev.coder.com" --app="code-server"
Running load test...

app url: https://paris.fly.dev.coder.com/@f0ssel/scaletest-1.dev/apps/code-server

[johnstcn]