Manifest releaser: Customize PR title

[phated]

Is your feature request related to a problem? Please describe.
When using the Manifest releaser, the PR title is always chore: release main. It seems like the PR title is configurable for the non-Manifest releaser. We'd like to customize the title of our PRs in a similar fashion.

Describe the solution you'd like
It'd be ideal to specify a PR title format in the release-please manifest file.

Describe alternatives you've considered
I've also considered writing a custom workflow that updates the PR title after release-please runs.

Additional context
Here's a Grain release PR: grain-lang/grain#872 - Notice that we have to remember to update the title to chore: Release Grain vX.X.X before we merge.

[chingor13]

This is probably possible to add an option like pull-request-title-pattern for the manifest itself.

We would only be able to inject the version if there is a configured root component (path of .), otherwise we wouldn't know which component to pick (and it would not necessarily be in every release).

[phated]

@chingor13 Maybe we can make it an option on the "." path? That's what we use currently.