Add ECDSA P-521 support for XDS interface

[tsaarni]

When Envoy connects to Contour over the xDS gRPC interface, it doesn't include secp521r1 in the TLS signature_algorithms extension because of BoringSSL’s default settings. As a result, the TLS handshake fails when using EC certificates with 521-bit private keys for the xDS interface.

This PR updates the Envoy bootstrap configuration to explicitly specify the list of supported signature algorithms, including secp521r1, enabling compatibility with these certificates.

Fixes #6997

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.74%. Comparing base (284fce7) to head (e64b4cf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6996      +/-   ##
==========================================
+ Coverage   80.72%   80.74%   +0.01%     
==========================================
  Files         131      131              
  Lines       19868    19884      +16     
==========================================
+ Hits        16039    16055      +16     
  Misses       3537     3537              
  Partials      292      292              

Files with missing lines	Coverage Δ
internal/envoy/v3/bootstrap.go	92.62% <100.00%> (+0.33%)	⬆️

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