Add opt-in mode for PR workflow

[kwannoel]

Basically if the label ci/opt-in is added, we can selectively run jobs.

Motivation for this is that some tests take a while to run / only run in ci. So some developers (me) will want to run it in ci for testing instead.

However instead of running all the steps in the pr workflow, I only need 2 or 3 tests to run.

So to speed up ci runtime + save ci costs, we can add this, to avoid having to modify the pull-request workflow as a workaround.

[kwannoel]

Not urgent, so removing from miletstone.