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)

Client usage docs: https://codeberg.org/harabat/pyforgejo#usage
Package in Fedora: https://packages.fedoraproject.org/pkgs/python-pyforgejo/python3-pyforgejo/

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Part of #866

[majamassarini]

You can read here: https://codeberg.org/harabat/pyforgejo that the new pyforgejo version will have breaking changes. I tried both the actual released version and the 2.0 branch. I can make authorized requests with the released version but I can’t with the 2.0 branch.

Waiting for an answer here.