Support groups of release PRs

[MarcoIeni]

Motivations

While I prefer "release on every PR", I'm unsure if that will work with all of the libraries we are dealing with and we'd likely need to be able to release subsets as needed.

From #1019 (comment)

Solution

We could have a pr-group config in the configuration file, so that release-plz opens 1 PR for each group of packages. Dependent packages, even if not specified in the configuration, should be included in the pr as well.
This is similar to the grouped update feature of dependabot.

[jbr]

Would this also include a "one release PR for each workspace crate" or is that a distinct use case?

[MarcoIeni]

I guess you can define one group for each crate :)
Having one PR for each crate, seems like a use case that could be enabled by this feature. But release-plz automatically figures out the groups, instead having the user to specify them.