Add manifest file attributes as action output(s)

### TL;DR

To help with using this workflow as an input to another job, I want to know some of the inputs on the way out, even if I'm using a manifest.
At a minimum, please output the `path`, which would correspond to the entry in `.release-please-manifest.json` that is involved in this release.

### Detailed design

My use-case is to use the `path` value to find an item in sfdx-project.json for Salesforce package development, and then to use values from that. If I have multiple packages being developed in one repo (our version of a monorepo) then each time RPAction runs, it should know which path in the manifest is being run, and should add that to its outputs.

The main goal here is to make an action that is analogous to uploading a package to npm, but for Salesforce. It will require adding a line to sfdx-project.json and editing some files, so I hope that any commit I make after RPAction but before that workflow finishes will be included in the same transaction.

### Additional information

I think this is a great tool, and I'm trying to standardize a way to make it easier to upload package versions for Salesforce - as easy as it is to make a new npm package version.

Some of my beginnings of my work are at https://github.com/dschach/packaging-action-test where I'm starting with a single-package repo and then will make a multi-package... which may require a pull-request on release-please, but I don't want to get ahead of myself.

Trace how the action reads .release-please-manifest.json and how action outputs are declared and populated. Add the manifest entry path to the outputs, then verify it is available to a subsequent job for the package selected by the release.

Written by the indexing model from the issue text.