Support for Bitbucket PRs in Mordred config

[SamuZad]

Amazing project! The GitHub and GitLab PR/MR + Issues integrations work really well

Would it be possible to add support Bitbucket PRs as well via Mordred config?

1. Bitbucket repos can be added as git repositories without any issues
2. Bitbucket is fairly popular, according to Bitrise
3. The Bitbucket API (while definitely limited in comparison to GitHub or GitLab), seems capable of providing the information
4. Other Atlassian products (Confluence + Jira) are supported in Mordred config

[vchrombie]

Hi @SamuZad

There was an initial attempt a few years ago, starting with perceval chaoss/grimoirelab-perceval#653 which was later moved to perceval-backends/grimoirelab-perceval-bitbucket. Feel free to checkout!