Support specifying a component in "Relase-As:", empty commits monorepos

It's convenient to be able to use an empty commit to specify a release version as described [here](https://github.com/googleapis/release-please/blob/main/docs/manifest-releaser.md). In the [OpenFeature](https://github.com/open-feature) project, we have a number of monorepos that use release-please, and sometimes we want to create a release for a _specific_ component in a monorepo without changes (releasing a 1.0 is a good example).

There doesn't seem to be a way to do this with an empty commit. Currently doing the above results in releasing all components at the specified version. It would be great if you could do something like add `Component: my-library` in the commit body to tell the manifest release which component you mean.

Apologies if this is already possible... I couldn't find doc about it.

I'd be interested in taking this on if there's agreement it's a reasonable addition.