Add pull-request-header and pull-request-footer to action's input

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**Is your feature request related to a problem? Please describe.**

Since v4, `pull-request-header` and `pull-request-footer` are only set in config JSON file. In JSON, it is not convenient to set multi-lines string. Until v3, I could read the content from file and pass it to the action's input like below;

```
- name: Get header
id: release-please-header
run: echo "content=$(cat release-please-header.md)" >> $GITHUB_OUTPUT
- use: googleapis/release-please@v3
with:
pull-request-header: ${{ steps.release-please-header.outputs.content }}
```

And, currently `config-file` is respected only when manifest json is loaded. Without manifest file, **there is no way** to configure pull-request-header and footer.

**Describe the solution you'd like**

Add `pull-request-header` and `pull-request-footer` inputs to release-please-actions.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by tracing how release-please-action reads action inputs and how config-file is handled when no manifest is loaded. Check the existing input definitions and related configuration flow. Done means pull-request-header and pull-request-footer can be supplied as action inputs, including multiline content, without requiring a manifest file.

Written by the indexing model from the issue text.