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.

[eshitachandwani]

The fields in xdsresource.Endpoint are set as an attribute of resolver.Endpoint in the balancers. Instead of setting it as a field while un-marshalling and setting as an attribute later, we want to directly set it as an attributes. This will remove the need for the xdsresource.Endpoint struct as we can add all the fields as attributes.

PR to remove HashKey field : #8844

[eshitachandwani]

Tasks :

• Remove HashKey : xds: remove HashKey field from xdsresource.Endpoint struct #8844
• Remove HealthStatus
• Remove Weight
• Remove Metadata
• Remove the complete xdsresource.Endpoint struct and replace with usages with resolver.Endpoint

[arjan-bal]

Do we foresee a scenario where all four fields will be used outside xDS? If not, could we instead store the values in a single attribute that holds all EDS information?

[easwars]

Hashkey is already used by ring_hash which can be used outside of xDS.

[easwars]

And there are some discussions about making pick_first be aware of endpoint weights (as part of the shuffling functionality required for soft affinity). So, that can also be outside of xDS.