Manually trigger an update for a specific dependency

Currently dependabot runs on a schedule, and by 'some magic' decides which of the outdated dependencies it will open a PR to update next. We tend to keep this limit to a relatively small number of PR's (2), and often need to take some extra time to ensure dependency changes don't affect our codebase's stability (not nearly enough test coverage to trust that in an automated way yet)

While obviously we can update dependencies manually when needed (which is our current workaround), we've grown quite fond of the way dependabot captures all of the relevant changelog/etc information into the PR when it opens one.

It would be great if we were able to 'trigger' dependabot in some way to automatically bump a specific outdated dependency (probably with the ability to bypass the 'max open PRs' limit for these 'manual bumps'). The only 'difference' here would be getting triggered manually, it would still open the PR and do everything else in the same way as it currently does.

No file, test, or entry point is named. Start by tracing how scheduled updates select dependencies and enforce the maximum open PR limit; done means a manual trigger can target one outdated dependency, open the usual update PR with its changelog details, and bypass that limit when requested.

Written by the indexing model from the issue text.