oauth2: Add support for configurable post_logout_redirect_uri

[funkluk]

Fixes #41644

Please bear with me, this is my first PR to the envoy-proxy project and any guidance is welcome.

Commit Message:
OIDC RP-initiated logout currently carries a hardcoded post_logout_redirect_uri. According to the OIDC specification, post_logout_redirect_uri is optional; implementers should be able to decide whether to include it and, if included, what value to use.
Two new, optional configuration settings allow specifying the desired URI or omitting the post_logout_redirect_uri parameter completely. If not defined, the current behaviour is kept, by adding the value <scheme>://<host>/ for the post_logout_redirect_uri parameter.

Risk Level: Low

Testing:
Added additional integration tests.
If more tests are required, please guide me what kind of tests and with an example.

Docs Changes:
No additional docs except the API description was done.

Release Notes:
see changelogs/current/new_features/oauth2__added_support_to_configure_post_logout_redirect_uri_parameter.rst
Not sure if this is the right way to add release notes or if I just have to add it to the current.yaml.

API changes:
Two new fields have be added to the oauth2 config:

• post_logout_redirect_uri (optional): allows specifying the required uri the OIDC OP should redirect back to
• disable_post_logout_redirect_uri (optional): if set to true, the post_logout_redirect_uri query parameter is omitted from the end_session_endpoint request.

[repokitteh-read-only]

Hi @funkluk, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #45367 was opened by funkluk.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @wbpcode
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #45367 was opened by funkluk.

see: more, trace.