otelconf: validate client key/cert symmetry for mTLS

[sandy2008]

Summary

• Error when a client key is provided without a client certificate in TLS config.
• Add a unit test for the client-key-only case.

Testing

• go build ./...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (32b20e5) to head (acc3d51).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8450   +/-   ##
=====================================
  Coverage   82.1%   82.2%           
=====================================
  Files        179     179           
  Lines      13690   13692    +2     
=====================================
+ Hits       11244   11257   +13     
+ Misses      2044    2033   -11     
  Partials     402     402           

Files with missing lines	Coverage Δ
otelconf/internal/tls/config.go	85.0% <100.0%> (+1.6%)	⬆️

... and 1 file with indirect coverage changes

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[sandy2008]

@dmathieu Thank you! Do you know when we can get it merged?

[dmathieu]

When we have a second approval.

[sandy2008]

@pellared @codeboten Could you help take a look :)? Thank you!!

[codeboten]

🎉