[wrossmorrow-pi-bot]

Summary

• Adds a plain gRPC load generator (LoadGen) that drives full ext_proc exchanges over a raw
  bidirectional gRPC stream to the SDK server, without envoy in the path, and reports per-exchange
  latency / extproc duration plus aggregate stats. Supports --addr, --iterations, --concurrency.
• Adds a TestingRequestProcessor ("testing" processor) mirroring the Go/Rust SDKs testers, which
  apply processing actions (append/add/overwrite/remove headers, clear/replace bodies, cancel)
  driven by a JSON action body, and supports the x-route-to-upstream override via clear-route-cache.

This is the last functional gap in the Milestone 4 (examples, harness, docs) alignment track, ahead
of the 0.3.0 release.

Notes

• Build/checks: spotlessCheck, compileJava, compileTestJava, test all pass.

Closes #0