project roles: make adding groups work

[valentijnscholten]

The Role (project role) resource has a method add_user that, according to its documentation and parameters, should be able to add either users or groups to a project role. The adding of groups was not (yet) implemented. This PR implements that.

Please note that JIRA server only supports either adding users XOR groups in a single request. So I have added a quick check to raise a clear error. The error message from JIRA is some vague 400 error about it being unable to parse the json request.

it's not perfect, but at least the method now works according to its docs.

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[studioj]

@valentijnscholten please merge in master, solve conflicts

[studioj]

thx => CI is running, if green it'll get reviewed

codacy is failing atm because of this https://app.codacy.com/gh/pycontribs/jira/pullRequest?prid=7435647 black is failing on resources.py https://github.com/pycontribs/jira/pull/884/checks?check_run_id=2657924451#step:8:47

[studioj]

stale for 4 months => close for now