Implement base for Forgejo support

[lbarcziova]

• create a directory for it in https://github.com/packit/ogr/tree/main/ogr/services
• prepare the file structure and classes with relevant inheritance (see how it is done for other services)
• use functionality from https://codeberg.org/harabat/pyforgejo/src/branch/main/pyforgejo/client.py for creating the service class
• make sure use_for_service decorator is applied for the new service class
• create the base exception class in ogr.exceptions
• add relevant tests (also for ogr.factory)

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Part of #866