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

Part of https://github.com/vitessio/vitess/issues/10697.

Current flags:

```
$ git grep -E "\bflag\.[A-Z]" -- go/vt/vtgate/grpcvtgateservice/*.go
go/vt/vtgate/grpcvtgateservice/server.go:       useEffective = flag.Bool("grpc_use_effective_callerid", false, "If set, and SSL is not used, will set the immediate caller id from the effective caller id's principal.")
```