xdsdepmgr: add functionality for cluster subscription

[eshitachandwani]

Part of A74 changes.
This PR adds the functions to subscribe to clusters which increases the refcount and to unsubscribe to the cluster which decreases the refcount. These functions will be used in a later PR to start cluster watches for dynamic clusters. Will also be used by xds_resolver to make sure PRCs subscribe to clusters they are routed to and that the watches for those clusters remain active.
RELEASE NOTES: None

[codecov]

Codecov Report

❌ Patch coverage is 76.81159% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.15%. Comparing base (572fdca) to head (8ef8677).
⚠️ Report is 34 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/xdsdepmgr/xds_dependency_manager.go	75.40%	11 Missing and 4 partials ⚠️
internal/xds/resolver/xds_resolver.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8792      +/-   ##
==========================================
- Coverage   83.31%   83.15%   -0.16%     
==========================================
  Files         414      414              
  Lines       32753    32805      +52     
==========================================
- Hits        27288    27279       -9     
- Misses       4064     4103      +39     
- Partials     1401     1423      +22     

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	80.04% <75.40%> (-0.38%)	⬇️

... and 46 files with indirect coverage changes

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[eshitachandwani]

@eshitachandwani Please update the PR description as per our offline discussion.

Done.