bench: add grpc vs resp3 throughput comparison

[kacy]

summary

• new bench-grpc.sh and bench-grpc.py for comparing gRPC vs RESP3 on standard SET/GET operations
• measures sequential and pipelined RESP3 (via redis-py) against gRPC unary calls (via ember-py)
• reports ops/sec and p50/p95/p99 latency for each test
• saves JSON results to bench/results/ for downstream processing
• supports --quick flag for a 10k-request sanity check

what was tested

• verified python harness correctly imports redis-py and ember-py
• confirmed JSON output structure is consistent with bench-vector.py patterns
• shell wrapper correctly starts server with both RESP and gRPC ports

design considerations

• gRPC streaming benchmarks deferred — ember's gRPC API currently uses unary RPCs only, so streaming would need server-side changes first
• pipeline batch size defaults to 100 (redis-py pipeline) to match typical usage