xds: Add cluster endpoint watchers to depndency manager

[eshitachandwani]

This is part of A74 implementation which add CDS/EDS/DNS watchers to the dependency manager. It also adds a temporary flag that is disabled by default so that it is not used in the current RPC paths , but enabled in the dependency manager tests.

RELEASE NOTES: None

[codecov[bot]]

Codecov Report

:x: Patch coverage is 81.55620% with 64 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 83.35%. Comparing base (6ed8acb) to head (09842ae).

Files with missing lines	Patch %	Lines
internal/xds/xdsdepmgr/xds_dependency_manager.go	85.09%	31 Missing and 14 partials :warning:
internal/xds/xdsdepmgr/watch_service.go	54.76%	16 Missing and 3 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8744      +/-   ##
==========================================
- Coverage   83.39%   83.35%   -0.05%     
==========================================
  Files         419      419              
  Lines       32566    32861     +295     
==========================================
+ Hits        27159    27390     +231     
- Misses       4023     4067      +44     
- Partials     1384     1404      +20     

Files with missing lines	Coverage Δ
internal/xds/resolver/xds_resolver.go	88.76% <100.00%> (+0.12%)	:arrow_up:
internal/xds/xdsdepmgr/watch_service.go	54.76% <54.76%> (-30.96%)	:arrow_down:
internal/xds/xdsdepmgr/xds_dependency_manager.go	84.15% <85.09%> (-0.75%)	:arrow_down:

... and 12 files with indirect coverage changes

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[easwars]

Can you also please check the codecov report here: https://github.com/grpc/grpc-go/pull/8744#issuecomment-3611618573 to see if there are any valid lines that are missing test coverage. Thanks.

[easwars]

I made a small refactoring commit to split the populateClusterConfigLocked method into smaller ones to handle updates for specific cluster types.

I also added the default case for the switch at the end. Please note that I changed it return true, nil, nil instead of false, nil, nil as it was doing earlier, because in this case, we have received the cluster resource. It just happens to be a type that we don't support. That shouldn't block us from sending an update if everything else in the tree is resolved (since we are anyways storing the error in the cluster config).