Get rid of xdsresource.Endpoint struct.

[Pranjali-2501]

We should completely get rid of the xdsresource.Endpoint struct, and encode all the information associated with the endpoint in the resolver.Endpoint struct.

After #8750, xdsresource.Endpoint struct will contain a slice of resolver.Endpoint instead of storing []Address.
And hostname field will be added in resolver.Endpoint.Attributes. Similarly, existing fields like Weight, Hashkey and HealthStatus can also be stored in attributes of resolver.Endpoint.