Add omniauth-oauth2-generic strategy

What does this MR do?

Adds the omniauth-oauth2-generic omniauth strategy

Are there points in the code the reviewer needs to double check?

Documentation

Why was this MR needed?

To provide an easier way to authenticate with various OAuth providers without writing and integrating new omniauth strategies for every one.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26744 (closed)