Serve complete TLS certificate chain by appending ca.crt

[Aditya7880900936]

What this PR does

Envoy Gateway previously served only the leaf certificate from tls.crt
and ignored ca.crt when building the served TLS certificate chain.

This PR:

• Extends the IR to carry optional CA certificates
• Appends ca.crt to the served certificate chain when present
• Keeps TLS validation behavior unchanged

Why this is needed

Incomplete certificate chains can cause client-side verification
and OCSP-related issues. This change aligns Envoy Gateway behavior
with common TLS expectations and cert-manager style secrets.

Backward compatibility

• No behavior change if tls.crt already contains the full chain
• No change to certificate validation or mTLS logic

Fixes #7804

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.66%. Comparing base (d88b5b5) to head (4ab0991).

Files with missing lines	Patch %	Lines
internal/xds/translator/listener.go	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8038      +/-   ##
==========================================
+ Coverage   73.65%   73.66%   +0.01%     
==========================================
  Files         237      237              
  Lines       35653    35662       +9     
==========================================
+ Hits        26259    26271      +12     
+ Misses       7530     7527       -3     
  Partials     1864     1864              

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