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

[lacolaco]

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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.