Ability to opt-in to auto-update with a label

[TSMMark]

Thanks for this neat GH action!

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I want to use it on PRs which are not auto_merge.

I'm thinking that this early return:

auto-update/src/index.ts

Line 92 in 8cf02f8

if (!pullRequest.auto_merge) {

could check if we've opted in via label, like:

!(pullRequest.auto_merge || pullRequest.labels.include?("auto-update:opt-in"))

Any concerns / Would you accept a PR for this?

Thanks