add AutoMerge: last run blocked by comment and Override AutoMerge: ignore blocking comments labels

[ericphanson]

This is the first RegistryCI feature that involves adding/removing labels programatically, as opposed to previous work that only reads them. This only affects the cron job. When it runs, it

1. Checks if the label "AutoMerge: last run blocked by comment" exists. If not it creates it. This will require new permissions, so this change is likely breaking
2. For each PR, if the PR has the label "Override AutoMerge: ignore blocking comments" then the PR does not count as blocked
3. if a PR is blocked, the label "Override AutoMerge: ignore blocking comments" is added (again, this needs new permissions)
4. if a PR is not blocked, the label "Override AutoMerge: ignore blocking comments" is removed (again, permissions)

Specifically, the permissions needed are: the github token by the cron job needs at least one of:

• "Issues" repository permissions (write)
• "Pull requests" repository permissions (write)

[DilumAluthge]

Unit tests on Julia 1.3 should be fixed now after JuliaRegistries/General#112623. I'll retry that job.

[ericphanson]

Is there a way to try the integration tests without merging?

[DilumAluthge]

We could add a workflow_dispatch trigger to the integration tests, which would allow us to trigger them manually.