Implement support for Forgejo issues

[lbarcziova]

• implement methods for the class inheriting from Issue
• use functionality from https://codeberg.org/harabat/pyforgejo/src/branch/main/pyforgejo/issue/client.py

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

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Part of #866

[betulependule]

It might be worth mentioning that the second link (to use functionality from) gives a 404 error.

[mfocko]

It might be worth mentioning that the second link (to use functionality from) gives a 404 error.

should be fixed