Add API endpoint to search for a specific deploy preview

[andrewnicols]

As a developer,
I want to find the build status for a build at a specific git commit.

At the moment I must run listSiteBuilds, and filter by the data.sha matching pull/[PR_NUMBER]/head, to get a build information containing a deploy_id which I can then query the status of.

It would be ideal if there were a listSitePreviews function accepting an optional commit head filter and/or pull request filter.