xds/googlec2p: Revert PR 8829 that removed SetFallbackBootstrapConfig

[easwars]

This API is still used internally in google3.

This reverts commit e6ca417.

RELEASE NOTES: none

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.26%. Comparing base (7985bb4) to head (97d491b).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/xdsclient/pool.go	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8869      +/-   ##
==========================================
+ Coverage   82.44%   83.26%   +0.81%     
==========================================
  Files         414      414              
  Lines       32751    32751              
==========================================
+ Hits        27002    27270     +268     
+ Misses       4094     4073      -21     
+ Partials     1655     1408     -247     

Files with missing lines	Coverage Δ
internal/xds/bootstrap/bootstrap.go	66.66% <100.00%> (+0.60%)	⬆️
internal/xds/xdsclient/pool.go	83.33% <90.47%> (+21.38%)	⬆️

... and 32 files with indirect coverage changes

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