Implement support for Forgejo user

[lbarcziova]

• implement methods for the class inheriting from GitUser
• implement the user property for the service class
• use functionality from https://codeberg.org/harabat/pyforgejo/src/branch/main/pyforgejo/api/user

Client usage docs: https://codeberg.org/harabat/pyforgejo#usage

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Part of #866