Rollup merge of #143695 - Urgau:waiting-on-author-new_draft, r=Kobzol

Auto-add `S-waiting-on-author` when the PR is/switches to draft state

This PR adds the `S-waiting-on-author` as a `new_draft` when the PR is/switches to draft state.

Related to rust-lang/triagebot#2102 & rust-lang/triagebot#2104

cc `@jieyouxu`
r? `@Kobzol`

Author: matthiaskrgr