xds: Add support for multiple addresses per endpoint

[arjan-bal]

This change adds support for endpoint resources with multiple addresses as part of implementing A61. The change reads the AdditionalAddresses field in the EDS resource. The parsing logic for the AdditionalAddresses is the same as that for the primary address. Validation is added to ensure that primary and additional addresses are unique across all localities. The additional addresses are added ONLY to ResolverState.Endpoints.Addresses when the flag is enabled. LB policies that use the ResolverState.Addresses (e.g. round robin) will not see any change in behaviour. Such LB policies will be updated later to delegate to the new leaf pickfirst LB policy to support endpoints.

A61 mentions that the code to read the new field should be protected by an environment variable flag named GRPC_EXPERIMENTAL_XDS_DUALSTACK_ENDPOINTS which will be removed once the dualstack support is proven stable. This PR adds a hardcoded boolen variable which will be read from the environment all LB policies handle endpoints.

This PR also reactors parts of clusterresolver to pass around a slice of resolver.Endpoints instead of a slice of address strings/addresses. This is required to retain the endpoint grouping. The endpoints list is flattened to an address list to ensure resolver states contain both addresses and endpoints.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.98%. Comparing base (36d5ca0) to head (64c5642).
Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7858      +/-   ##
==========================================
+ Coverage   81.69%   81.98%   +0.28%     
==========================================
  Files         374      379       +5     
  Lines       37978    38261     +283     
==========================================
+ Hits        31028    31367     +339     
+ Misses       5638     5579      -59     
- Partials     1312     1315       +3     

Files with missing lines	Coverage Δ
balancer/weightedroundrobin/weightedroundrobin.go	100.00% <100.00%> (ø)
internal/testutils/xds/e2e/clientresources.go	98.18% <100.00%> (+0.04%)	⬆️
...ternal/balancer/clusterresolver/clusterresolver.go	68.08% <100.00%> (+1.12%)	⬆️
...internal/balancer/clusterresolver/configbuilder.go	90.90% <100.00%> (+0.16%)	⬆️
...rnal/balancer/clusterresolver/resource_resolver.go	94.44% <100.00%> (ø)
.../balancer/clusterresolver/resource_resolver_dns.go	72.09% <100.00%> (+6.50%)	⬆️
xds/internal/balancer/wrrlocality/balancer.go	65.26% <100.00%> (+1.13%)	⬆️
xds/internal/internal.go	68.96% <100.00%> (+3.58%)	⬆️
...ds/internal/xdsclient/xdsresource/unmarshal_eds.go	94.57% <100.00%> (+0.21%)	⬆️

... and 48 files with indirect coverage changes

[easwars]

Haven't looked at the clusterresolver changes yet.

[easwars]

Sorry for the delay in getting back to this PR.