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]

Codecov Report

❌ Patch coverage is 72.69625% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.22%. Comparing base (4c27cc8) to head (4862844).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/xdsdepmgr/xds_dependency_manager.go	74.78%	33 Missing and 26 partials ⚠️
internal/xds/xdsdepmgr/watch_service.go	64.40%	18 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8744      +/-   ##
==========================================
- Coverage   83.28%   83.22%   -0.07%     
==========================================
  Files         418      418              
  Lines       32367    32656     +289     
==========================================
+ Hits        26958    27178     +220     
- Misses       4034     4073      +39     
- Partials     1375     1405      +30     

Files with missing lines	Coverage Δ
internal/xds/xdsdepmgr/watch_service.go	63.73% <64.40%> (-21.98%)	⬇️
internal/xds/xdsdepmgr/xds_dependency_manager.go	76.20% <74.78%> (-8.71%)	⬇️

... and 23 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.