feat: allow onDemand configuration for release PRs

[chingor13]

This feature allows a repo to configure onDemand: true which will only create the release PRs when you use the release-please:force-run label. It will tag the releases on pushes to the release branch (e.g. main).

This is useful for monorepos that choose when they want to run releases (i.e. they do not always want a release PR as soon as a feature is merged). Additionally, it should reduce the sheer number of API calls the app makes for monorepos that have a set release schedule.

To configure, you can add onDemand: true to your .github/release-please.yml. Example:

manifest: true
onDemand: true
branches:
- branch: 1.x
  manifest: true
  onDemand: true

Note: this option makes it so that your release PR could be "out of date" with the latest code in the release branch, so maintainers will need to pay close attention.

[dazuma]

Yes please! (From the Ruby monorepo owner who broke owlbot yesterday due to quota issues stemming from multiple updates to hundreds of release PRs.)