Add grpchealth client

[emcfarlane]

Add a Client type with Check and Watch methods that wrap the internal generated protobuf types. Callers use only the library's own CheckRequest, CheckResponse, and Status types, so there is no proto registration conflict when the same process also imports google.golang.org/grpc/health/grpc_health_v1. Watch returns an iter.Seq2 iterator instead of a bespoke stream wrapper or channel to avoid a custom struct or goroutine.

Closes #72