Add support for using an alternative git hosting platform: forgejo

ATM we rely on github for hosting resultant repositories.

I am seriously considering spinning up an instance of the [forgejo-aneksajo](https://codeberg.org/forgejo-aneksajo/forgejo-aneksajo):

- https://github.com/dandi/dandi-utils/issues/14

But I would like not to simply replace all github calls with forgejo ones, but rather make it configurable, so any particular deployment of DANDI infrastructure and this `backups2datalad` could decide between available choices.

I think overall it would entail the following:

- Investigate if forgejo API endpoints are sufficient to support all implemented here actions against github
- Refactor code to abstract away interaction with remote service endpoints
- Add support for forgejo
- Add functionality to "migrate" from github to forgejo
- an "alternative" could be to somehow support multiple such remote git backends, might make "migration" to be just a matter of adding one more and eventually removing the "old" one