Implement additional fields syncing for pull requests

[psss]

Currently some syncing features are limited to issues only. It would be nice to synchronize all essential keys for pull requests as well. Especially the following fields would be very useful:

• title
• description
• comments
• github_markdown
• github_project_fields

Copy pasting relevant comment from the chat:

I think we all agree that the tool could and should sync “all the things” for PRs like it does for Issues.  The challenges are centered around trying to avoid replicating/duplicating code (i.e., can we support PRs and Issues with the same code so that we get uniform behavior without undue maintenance burden), how do we provide sufficient behavioral control via the config files (e.g., do we provide separate controls over PRs and Issues), and how do we make separate controls work with common code.  I.e., the details…