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

[ajm188]

Part of https://github.com/vitessio/vitess/issues/10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/vttablet/tabletconn/*.go
go/vt/vttablet/tabletconn/tablet_conn.go:       TabletProtocol = flag.String("tablet_protocol", "grpc", "how to talk to the vttablets")