[Security] Supporting negotiation of certs based on signing algorithm

[gtcooke94]

This PR fixes issues where gRPC-Go would not correctly support the negotiation of certs based on signing algorithm when multiple are provided. There is no API change.

In advancedtls.go and sni.go, we now call buildGetCertificates unconditionally as this is where we select the certificate based on the desired SNI and signature algorithm.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.10%. Comparing base (1ecbd86) to head (0069064).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9321      +/-   ##
==========================================
- Coverage   83.11%   83.10%   -0.02%     
==========================================
  Files         423      423              
  Lines       35231    35236       +5     
==========================================
- Hits        29284    29283       -1     
- Misses       4432     4442      +10     
+ Partials     1515     1511       -4     

see 44 files with indirect coverage changes

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[easwars]

@gtcooke94 : Could you please get someone from the security team to review this before the grpc team starts looking at this.

Also, is there a gRFC/design that could be linked in the PR description. It is currently quite empty.

[gtcooke94]

Will get the security team to review soon once I finalize a few items

There's no public API change for this, but it's pre-work for post-quantum authenticity support. In fact, it's more of a bug in how we were handling things before, it's just that the use-case of negotiating the signature algorithm was never really used.