New badge: Average issue response time

:clipboard: **Description**
Usually a maintainer comments in an issue, or the issue is closed. The reponse time for an issue can be `comment or close time - issue create time`.

:link: **Data**

GitHub API
- https://docs.github.com/en/rest/reference/issues#get-an-issue
- https://docs.github.com/en/rest/reference/issues#list-issue-comments
- https://docs.github.com/en/rest/reference/issues#get-an-issue-comment

:microphone: **Motivation**

People can put the badge in their README. When people make an issue, they will know around how much time they'll have to wait until someone responds.

Start with the GitHub REST endpoints listed in the issue, checking which issue and comment timestamps are available and how the average should be defined. Done means a Shields badge can expose the average time from issue creation until a maintainer comment or closure, using the agreed GitHub data for README use.

Written by the indexing model from the issue text.