Switch flag definitions to be on pflag instead of flag in `package go/vt/grpcclient`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/grpcclient/*.go
go/vt/grpcclient/client.go:     keepaliveTime         = flag.Duration("grpc_keepalive_time", 10*time.Second, "After a duration of this time, if the client doesn't see any activity, it pings the server to see if the transport is still alive.")
go/vt/grpcclient/client.go:     keepaliveTimeout      = flag.Duration("grpc_keepalive_timeout", 10*time.Second, "After having pinged for keepalive check, the client waits for a duration of Timeout and if no activity is seen even after that the connection is closed.")
go/vt/grpcclient/client.go:     initialConnWindowSize = flag.Int("grpc_initial_conn_window_size", 0, "gRPC initial connection window size")
go/vt/grpcclient/client.go:     initialWindowSize     = flag.Int("grpc_initial_window_size", 0, "gRPC initial window size")
go/vt/grpcclient/client_auth_static.go: credsFile = flag.String("grpc_auth_static_client_creds", "", "when using grpc_static_auth in the server, this file provides the credentials to use to authenticate with server")
go/vt/grpcclient/snappy.go:     compression = flag.String("grpc_compression", "", "Which protocol to use for compressing gRPC. Default: nothing. Supported: snappy")