xds/resolver: add a way to specify the xDS client to use for testing purposes

[easwars]

#a71-xds-fallback
#xdsclient-refactor
Addresses #6902

This PR adds a way to specify the exact xDS client to be used by the xDS resolver. Currently we only have a way of specifying the bootstrap configuration to use.

This change is required for fallback tests where we need to be able to configure xDS clients with options (like idle authority timeout) that are not part of the bootstrap configuration.

Subsequent PRs will use this functionality.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.29%. Comparing base (8212cf0) to head (5355798).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
xds/internal/resolver/xds_resolver.go	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7771      +/-   ##
==========================================
- Coverage   81.40%   81.29%   -0.11%     
==========================================
  Files         368      368              
  Lines       36768    36775       +7     
==========================================
- Hits        29932    29898      -34     
- Misses       5607     5643      +36     
- Partials     1229     1234       +5     

Files with missing lines	Coverage Δ
xds/internal/resolver/xds_resolver.go	77.53% <33.33%> (-1.40%)	⬇️

... and 19 files with indirect coverage changes

[purnesh42H]

This change is required for fallback tests where we need to be able to configure xDS clients with options (like idle authority timeout) that are not part of the bootstrap configuration.

Why do we not have that in bootstrap configuration?

[purnesh42H]

lgtm

[easwars]

Why do we not have that in bootstrap configuration?

That is not something that is visible to our users. This is not a configurable option. The only reason I want to make it configurable is for testing. And I prefer e2e tests, so we need to way to inject this.