credentials/tls: Splitting host and port of authority header

[Atul1710]

Fixes: #8719

Splits the host and port in the HTTP2 :authority header in ValidateAuthority before calling VerifyHostname. Added test cases to check multiple different types of values possible for :authority

Release notes:

• tls: strip port before validating authority override.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Atul1710 / name: Atul Suresh Kumar (7baceed, cf93c27, dcc3b9d)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (cdbafd3) to head (cf93c27).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8726      +/-   ##
==========================================
+ Coverage   83.25%   83.33%   +0.07%     
==========================================
  Files         419      419              
  Lines       32427    32432       +5     
==========================================
+ Hits        26997    27026      +29     
+ Misses       4047     4034      -13     
+ Partials     1383     1372      -11     

Files with missing lines	Coverage Δ
credentials/tls.go	91.53% <100.00%> (+2.56%)	⬆️

... and 28 files with indirect coverage changes

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[arjan-bal]

The existing tests for authority override are located here:

1. Tests for successful override
2. Tests for invalid override

Could you please add a new test case to the list of successful cases? For the invalid override tests, it would be great if you could refactor them into a table-driven test (similar to the success cases) and include a case for the changes in this PR. This will help avoid code duplication.