Support multiple git hosting providers

[koppor]

Users might use another Git provider than GitHub

Examples:

• GitHub enterprise (e.g., https://github.tik.uni-stuttgart.de/)
• GitLab
• Custom GitLab hostings
• CodeBerg

and much much more.

This should be supported.

Needs to be refined.

Follow-up to JabRef#13677.

[wanling0000]

TODO: Support provider-based Git credential storage

In the future, JabRef should support storing Git credentials per provider (e.g., GitHub, GitLab, CodeBerg).

• In JabRef's preferences, have store for provider/username/password - similar to org.jabref.gui.preferences.websearch.WebSearchTabViewModel
  One can store "provider" using "domain".