Add a pr-run-mode that can handle projects with different versions

Feature request: make it so the release workflow for a PR can cope with a workspace that has projects that have different version numbers. Something like `pr-run-mode="plan-by-project"`.

Rather than reporting “too many unrelated apps in your workspace to coherently Announce,” run `cargo dist plan` with the tag and version of each project.

Example: here is a PR where we were trying to update the version of one project in a workspace:
- https://github.com/dfinity/dfx-extensions/pull/100 (see [first commit](https://github.com/dfinity/dfx-extensions/pull/100/commits/63548c17a77aca1bb56fef0c59748ac188a483b3))

Since we were using the default `pr-run-mode="plan"`, we got this error:

> × There are too many unrelated apps in your workspace to coherently Announce!
> help: Please either specify --tag, or give them all the same version
>
> Here are some options:
>
> --tag=v0.3.1 will Announce: nns
> --tag=v0.4.0 will Announce: sns
>
> you can also request any single package with --tag=nns-v0.3.1

It's a PR, so we can't specify a tag, but from the error message we can see that `cargo dist plan` already knows which tags we _should_ specify for every project.

We ended up having to update the versions of both projects. This was fine; we were going to update both anyway; but it won't always be the case that project versions are in lockstep.

I would like for the release.yml workflow in this case to effectively run `cargo dist plan --tag=-v` for every project in the workspace, which in this example would be:
```
cargo dist plan --tag=nns-v0.3.1
cargo dist plan --tag=sns-v0.4.0
```

In other words: I’m opening a PR to update the version of one project in a workspace. I’m going to follow it up with `cargo dist plan --tag=-v && cargo release -p `. I'd like the release workflow to do what it can to verify that this will work, even if that means running `cargo dist plan` for every project.

As far as I can see, the only workaround is to set `pr-run-mode="skip"`.

Start with the release.yml workflow and its existing pr-run-mode="plan" handling. Read the cargo dist plan behavior described in the issue, including the project-specific tags shown in the error, and compare it with cargo release -p usage. Done means the workflow can validate each project in a mixed-version workspace without requiring all versions to match.

Written by the indexing model from the issue text.