feat(driver): gRPC driver (v1.1.0)

[lewta]

Summary

Adds a type: grpc driver for generating gRPC traffic — the first non-HTTP protocol from the v1.1.0 milestone.

• No .proto files required — uses server reflection to discover request/response types at runtime, then marshals the JSON body to protobuf automatically
• URL scheme selects transport: grpc://host:port/Service/Method (plaintext), grpcs:// (TLS)
• gRPC status codes mapped to HTTP-like equivalents so the engine's error classifier, backoff, and Prometheus metrics work unchanged across all driver types
• Connections and method descriptors cached per address+TLS mode — reflection is called only once per method
• New config fields: grpc.body, grpc.timeout_s, grpc.tls, grpc.insecure on TargetConfig and TargetDefaultsConfig

Config example

targets:
  - url: grpc://api.example.com:50051/helloworld.Greeter/SayHello
    type: grpc
    weight: 10
    grpc:
      body: '{"name": "world"}'
      timeout_s: 15

Test plan

• 10 driver tests: status mapping, health check (JSON body + empty body), connection reuse, NOT_FOUND status (404), unknown method error, no-reflection error, invalid URL, cancelled context, gRPC status FromError integration
• Full test suite green (go test -race ./...)
• go vet ./... clean
• gofmt -s clean

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