[WIP] Add OAuth2 flow

[koppor]

Follow-up to https://github.com/JabRef/blog.jabref.org/pull/128

This just follows https://connect2id.com/products/nimbus-oauth-openid-connect-sdk/examples/oauth/authorization-request

Steps to test

TBD

Mandatory checks

• [x] I own the copyright of the code submitted and I license it under the MIT license
• [ ] I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• [ ] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.