xds: store server config for LRS server in xdsresource.ClusterUpdate

[easwars]

Summary of changes:

• Add server config to the xdsresource.DecodeOptions struct. This field will contain the configuration of the xDS server from where the resource was received.
  • The authority struct inside the xdsclient sets up this field when it receives a Cluster resource and passes it on to the data model layer for parsing and decoding.
• Store the server config of the LRS server in the xdsresource.ClusterUpdate struct, instead of an enum representing whether LRS is ON.
  • The data model layer populates this field if it sees LRS being enabled in the received Cluster resource.
• Change CDS LB policy to simply copy the LRS server config from the ClusterUpdate struct to the child policy config.
• Update tests.
• Remove a TODO which has already been taken care of.

#a71-xds-fallback

Fixes #6896

RELEASE NOTES: none

[dfawley]

IIRC we're still missing something because we aren't supporting LRS for aggregate clusters?

[easwars]

As mentioned in #7192, we currently don't support LRS for non-EDS clusters, and according to Mark, that is a bug. And with regards to aggregate clusters, Mark mentioned that the expected behavior is specified in A75, but Go hasn't implemented A74 and A75 yet.

So, in this PR, I didn't want to add LRS support for non-EDS clusters. I will do that as part of #7192.

[easwars]

Thanks for the review.

[easwars]

As mentioned in #7192, we currently don't support LRS for non-EDS clusters, and according to Mark, that is a bug. And with regards to aggregate clusters, Mark mentioned that the expected behavior is specified in A75, but Go hasn't implemented A74 and A75 yet.

So, in this PR, I didn't want to add LRS support for non-EDS clusters. I will do that as part of #7192.