Allow queries in `githubPullRequests.queries` to query all repos or specific repos

[alexr00]

Currently githubPullRequests.queries only lets you query against the currently open repos. We could also allow you to query against specific repos or all repos.

Context: #3429 (comment)

[djobin-cap]

Hi!
I would really like it, if the githubPullRequests.queries could somehow show me every PR that are waiting for my review in my Organization (or at least in the open repos) ... but all at once, in a single tree view, without the need to browse every repos trees.

The "same" view as when you go to https://github.com/pulls/review-requested