Add support for writing summary comment to pull request

It would be nice if there would be a configuration to enable Cake.Issues to write a summary of the issues as comment to the pull request.

On multiple runs the comment should be updated instead of multiple comments created.

This should be implemented for all pull request systems:

- Cake.Issues.PullRequests.AzurePipelines

Review the Cake.Issues.PullRequests.AzurePipelines integration first, then trace how pull-request comments are currently handled. Done means a configuration enables an issue summary comment, repeated runs update that comment instead of creating duplicates, and the behavior is implemented for all pull-request systems in scope.

Written by the indexing model from the issue text.