Add GitHubPRComment DevOps task

[ceciliaavila]

Description

This PR moves the GitHubPRComment task from southworks/SOUTHWORKS-azure-pipelines-tasks to this repository to be re-published as a Microsoft task.

This task receives plain text files and uses the content to create a GitHub comment on a Pull Request. It must be configured with a GitHub User Token, a repository name, a PR number, and the file path to get the text for the comment.

It's currently used in the BotBuilder-DotNet-CI-PR pipeline along with the API Compat task to post messages in the PRs that introduce compatibility issues.

Specific Changes

• Adds the GitHubPRComment source, task definition files, and documentation.

[ceciliaavila]

Hi @mrivera-ms, we updated the PR's description with the info you requested. Thanks!