GRPCServer runs a *grpc.Server without depending on grpc

[pior]

Adds the second server wrapper that ServerOption was designed for.

GRPCServer(addr, s, opts...) runs a *grpc.Server. It takes an interface with the Serve, GracefulStop and Stop methods, so the module doesn't depend on grpc. It behaves and logs like HTTPServer: listening, draining, then drained, drain timed out or drain failed.

• Drain: on cancellation it calls GracefulStop. If DrainTimeout expires first, it calls Stop and returns server shutdown: context deadline exceeded.
• One implementation: HTTPServer and GRPCServer share an unexported server core built from a serve function and a shutdown function.
• addr is required: a *grpc.Server has no address. Listener(ln) overrides it.
• HTTPServer still reads server.Addr when Run is called, as on main, so an Addr set after construction is still used.

Integration module

integration/grpc is a separate Go module that tests GRPCServer against a real *grpc.Server: a clean drain, and a drain timeout on a never-ending stream that falls back to Stop. grpc stays out of the runnable module, even as a test dependency. CI runs it as a matrix job (lint, go mod verify, go mod tidy -diff, go test -race), and bud integration runs it locally. gomoddirectives allows its local replace.

The main test job now also runs go mod download and go mod verify.