gRPC support

[FumingPower3925]

Problem

Cannot benchmark gRPC servers. The load generator only supports HTTP/1.1 and HTTP/2.

Solution

Implement a gRPC client that satisfies the exported Client interface (#7):

• Support unary RPC calls
• Accept protobuf message descriptors or raw bytes
• Support reflection-based discovery (optional)

Blocked On

• Export Client interface for extensibility #7 (Export Client interface)

Acceptance Criteria

• gRPC client implements Client interface
• Supports unary RPCs
• Configurable protobuf payloads
• Tests with gRPC test server

Future

This is a post-v1.0.0 feature.