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

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/vt/binlog/grpcbinlogplayer/*.go
go/vt/binlog/grpcbinlogplayer/player.go:        cert = flag.String("binlog_player_grpc_cert", "", "the cert to use to connect")
go/vt/binlog/grpcbinlogplayer/player.go:        key  = flag.String("binlog_player_grpc_key", "", "the key to use to connect")
go/vt/binlog/grpcbinlogplayer/player.go:        ca   = flag.String("binlog_player_grpc_ca", "", "the server ca to use to validate servers when connecting")
go/vt/binlog/grpcbinlogplayer/player.go:        crl  = flag.String("binlog_player_grpc_crl", "", "the server crl to use to validate server certificates when connecting")
go/vt/binlog/grpcbinlogplayer/player.go:        name = flag.String("binlog_player_grpc_server_name", "", "the server name to use to validate server certificate")