Implement support for Forgejo PRs

[lbarcziova]

• implement methods for the class inheriting from PullRequest
• use functionality from https://codeberg.org/harabat/pyforgejo/src/branch/main/pyforgejo/api/repository

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

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Part of #866

[olamidepeterojo]

Hello @lbarcziova, i would love to contribute to this