xdsdepmgr: add functionality for cluster subscription

[eshitachandwani]

Part of A74 changes.
This PR add the functions and functionalities to be used for cluster subscription for cluster refcounts and also for dynamic cluster subscription. These functions will be used in subsequent PRs.
RELEASE NOTES: None

[codecov]

Codecov Report

❌ Patch coverage is 67.34694% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (629ef39) to head (2efe5da).

Files with missing lines	Patch %	Lines
internal/xds/xdsdepmgr/xds_dependency_manager.go	63.41%	12 Missing and 3 partials ⚠️
internal/xds/resolver/xds_resolver.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8792      +/-   ##
==========================================
+ Coverage   83.22%   83.29%   +0.07%     
==========================================
  Files         417      417              
  Lines       32920    32963      +43     
==========================================
+ Hits        27397    27457      +60     
+ Misses       4108     4100       -8     
+ Partials     1415     1406       -9     

Files with missing lines	Coverage Δ
internal/xds/resolver/xds_resolver.go	88.70% <87.50%> (-0.06%)	⬇️
internal/xds/xdsdepmgr/xds_dependency_manager.go	79.74% <63.41%> (-0.68%)	⬇️

... and 24 files with indirect coverage changes

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[arjan-bal]

Some minor comments, otherwise LGTM!

Questions regarding the cluster subscription API.

[arjan-bal]

LGTM!