BE: Add logout with query parameters to generic OAuth provider

[oldium]

The logout URL is taken from auto-detected endpoint from the provider's well known configuration address, but can be overridden by custom parameter logout-url. Query parameters are optional and are added only when the corresponding key is present. Default redirect URL is a base URL and can be overridden in custom parameters.

Also the final slash from the redirect URL can be removed when redirect-url-strip-slash is set to true/yes/on/1.

Example with all custom parameters:

custom-params:
  type: oauth
  logout-url: https://example.com/sso/logout
  client-id-key: clientId
  redirect-url-key: returnTo
  redirect-url: https://example.com/kafka-ui
  redirect-url-strip-slash: true

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

I generalized original Cognito logout code and created common ancestor. I added new OAuth logout handling, which is generic enough to handle Cognito logout too, but I kept Cognito for simplicity of
configuration.

Is there anything you'd like reviewers to focus on?

I changed the calculation of redirect URL after logout to have the context path. For normal installation there is no change in behaviour (context path is root - "/").

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• Manually (please, describe, if necessary)

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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