Add "release new crate version" workflow

[joshlf]

This can be triggered manually via the GitHub web UI, and will generate a PR which updates the crate versions in both Cargo.toml files.

[joshlf]

I tested this manually by temporarily adding a pull_request trigger to this workflow, which generated #1173.

Note that the GitHub Actions meta-variable ${{ github.event.inputs.version }} is not set when triggered by pull_request, so the fact that the version string is empty in that PR is expected given the testing methodology.

[jswrenn]

It'd also be nice to parameterize this by branch so we could use the same process for releases to older version trains, but this looks like a great start to me!

[joshlf]

Now that this is merged, here's a better example of a PR - this one generated via the manual trigger UI: #1178