Replace contour-authserver in e2e with gRPC server running in test suite

[tsaarni]

This PR proposes dropping contour-authserver as a dependency for the e2e test suite and replacing it with a test server stub that runs directly within the test case.

This approach makes the external authorization tests self-contained, allowing the authorization server to be easily programmed to return specific responses or assert request parameters right inside the test case.

This is similar to the HTTP-based authorization server I mentioned here: #7418 (comment)

configure Envoy to connect to the host via a Service and EndpointSlice, and then set up an HTTP server as part of e2e test suite, giving the test case full control to run assertions.

Additionally, contour-authserver has not been maintained for a long time, with its last release in 2023. It might be better to archive that project altogether.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.35%. Comparing base (34d2ab1) to head (7425508).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7597   +/-   ##
=======================================
  Coverage   82.35%   82.35%           
=======================================
  Files         130      130           
  Lines       15873    15873           
=======================================
  Hits        13073    13073           
  Misses       2513     2513           
  Partials      287      287           

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