Feature Request: support fetching release-please-config.json from the file system

**Is your feature request related to a problem? Please describe.**
We are using shared workflow as an wrapper for the workflow for easy maintenance and upgrade. Back in v3 of the workflow, we can provide `inputs` with `default` values to the shared workflow, then the `inputs` can be modified per repository when necessary. Basically, we have a shared config with default settings and the option to modify during runtime:
```
- uses: googleapis/release-please-action@v3
id: release
with:
default-branch: ${{ inputs.master_branch }}
release-type: ${{ inputs.release_type }}
package-name: ${{ github.event.repository.name }}
include-v-in-tag: ${{ inputs.include_v_in_tag }}
pull-request-title-pattern: ${{ inputs.pull_request_title_template }}
path: ${{ inputs.path }}
monorepo-tags: ${{ inputs.monorepo-tags }}
```

In v4, since all advanced configurations now require a config file, we are trying to dynamically generate the config file during runtime, with the above configurations roughly converted to:
```
- name: Prepare release-please-config.json
id: prepare-config
run: |
cat > release-please-config.json <