Cherry pick #7980 to v1.70.x

[arjan-bal]

Original PR: #7980
Related Issues: #434, #7922

RELEASE NOTES:

• experimental/credentials: Experimental transport credentials are added which don't enforce ALPN. These will be removed in upcoming grpc-go releases. Users must not rely on these credentials directly. Instead, they should either vendor a specific version of gRPC or copy the relevant credentials into their own codebase if absolutely necessary.

[codecov]

Codecov Report

Attention: Patch coverage is 81.92771% with 30 lines in your changes missing coverage. Please review.

Project coverage is 82.15%. Comparing base (62b9185) to head (f996ccf).
Report is 1 commits behind head on v1.70.x.

Files with missing lines	Patch %	Lines
experimental/credentials/tls.go	75.80%	26 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.70.x    #8012      +/-   ##
===========================================
+ Coverage    81.99%   82.15%   +0.16%     
===========================================
  Files          381      384       +3     
  Lines        38542    38706     +164     
===========================================
+ Hits         31602    31799     +197     
+ Misses        5616     5598      -18     
+ Partials      1324     1309      -15     

Files with missing lines	Coverage Δ
credentials/tls.go	86.57% <100.00%> (ø)
experimental/credentials/internal/spiffe.go	100.00% <100.00%> (ø)
experimental/credentials/internal/syscallconn.go	100.00% <100.00%> (ø)
experimental/credentials/tls.go	75.80% <75.80%> (ø)

... and 24 files with indirect coverage changes