create reaction for pull request comment

[shiftkey]

Link: https://developer.github.com/v3/reactions/#create-reaction-for-a-pull-request-review-comment

This is a new endpoint which should live in IPullRequestReviewCommentClient

POST /repos/:owner/:repo/pulls/comments/:id/reactions

Example request payload:

{
  "content": "heart"
}

As these are a limited subset of emojis, the request object should use an enumeration of values - rather than a regular string.

Example response payload:

{
  "id": 1,
  "user_id": 1,
  "content": "heart"
}

• define classes for payload in Octokit project and .\build FixProjects to make them available everywhere
• add new method to IReactionClient
• add new method to IObservableReactionClient
• implement both interfaces and ensure preview header is sent: application/vnd.github.squirrel-girl-preview
• unit tests to verify behaviour
• integration test to confirm code works against real API