Add support for pull request approval

Original issue: https://github.com/cake-contrib/Cake.Prca/issues/54

Add functionality that the pull request implementation can approve the pull request.

- Should be option for pull request implementation to implement (maybe introduce some call where the main addin asks for capabilities)
- Add flag to `ReportCodeAnalysisIssuesToPullRequestSettings`
- Cake.Issues.PullRequests addin would determine status (maybe we need to add some settings to define the criteria, eg on priority, etc)
- Enhance `IPullRequestSystem` with method to approve or decline a PR
- Virtual implementation in `PullRequestSystem` which doesn't do anything

Start by tracing ReportCodeAnalysisIssuesToPullRequestSettings, the Cake.Issues.PullRequests addin, IPullRequestSystem, and its PullRequestSystem implementation. Clarify how capabilities and approval criteria should be configured, then verify that supported implementations can approve or decline a pull request while the virtual implementation remains a no-op.

Written by the indexing model from the issue text.