protobuf: upstream_bind_config is a core.v3.BindConfig instead of an UpsteamBindConfig

Description: I'm not sure if it's just "misspelt" or it has to be like this, but cluster.upstream_bind_config is a core.v3.BindConfig. I think it should be a UpstreamBindConfig. This way UpstreameBindConfig message is totally unused. And using the current state does nothing.

Additional questions: How exactly should this field work? With this upsteam_bind_config field should I be able to tell envoy which port_value should it use when sending datagrams to the upstream host? I mean we want to control the port value from our service gets its datagrams. Instead of letting envoy open a socket from a random port, we want to tell which specific port to use when connecting to an upstream host. Is it possible?