[github pull requests] Add support for github pull requests

[aswanipranjal]

Currently grimoire-elk supports parsing github issues data and is unable to parse pull requests data if fetched using perceval.
This issue is regarding added support to calculate enriched data for github pull requests.

To start, I think we have the following points to consider:-

• We can fetch the pull_request data from perceval using the --category pull_request flag
• We can have a different elasticsearch raw index to store the PR data fetched from perceval and similarly a different elasticsearch enriched index to store the PR enriched data
• We need a github_pr backend for the raw as well as enriched index, in elk