bootstrap: add String method to ServerConfigs type

[easwars]

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 81.86%. Comparing base (86135c3) to head (dbf5688).
Report is 9 commits behind head on master.

Files	Patch %	Lines
internal/xds/bootstrap/bootstrap.go	18.18%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7537      +/-   ##
==========================================
+ Coverage   81.82%   81.86%   +0.04%     
==========================================
  Files         360      360              
  Lines       27533    27508      -25     
==========================================
- Hits        22528    22520       -8     
+ Misses       3804     3793      -11     
+ Partials     1201     1195       -6     

Files	Coverage Δ
internal/testutils/xds/e2e/bootstrap.go	64.00% <100.00%> (ø)
internal/xds/bootstrap/bootstrap.go	70.47% <18.18%> (-1.88%)	⬇️

... and 28 files with indirect coverage changes

[arvindbr8]

Overall LGTM, unless you think the stringer method might benefit from unit tests.

Also on a side note: Should we modify NewContentsForTesting to Unmarshal to ServerConfigs directly?

[easwars]

Also on a side note: Should we modify NewContentsForTesting to Unmarshal to ServerConfigs directly?

That's a good idea. Did that.

[easwars]

Overall LGTM, unless you think the stringer method might benefit from unit tests.

Thought about it. It will mostly be used only for logging and/or debugging. So. I'm not too worried about not having unit tests for that. In fact, we don't want anything to rely on this output being stable.