Add username_key setting to configure python-social-auth OIDC

[marius-mather]

This PR adds the USERNAME_KEY setting to the OIDC config, which is used by python-social-auth to extract the username. We are implementing an OIDC login system across multiple platforms and need to configure this so we can have a Galaxy-specific username.

AuthnzManager and PSAAuthnz did not seem to have existing unit tests so I've added basic tests of parsing the XML config to check that values are passed through correctly.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.