Add the input for GitHub action to specify release message pattern

Currently, there is no way to change the release message.
Thus, the user cannot add static text before/after changelog or use bumped version anyhow.

It seems to me a template with variables could help many users. E.g.:
```
- uses: GoogleCloudPlatform/release-please-action@v2
with:
release-type: node
package-name: release-please-action
release-message: |
The community keeps growing, and we'd love to see you there!

- Link to slack
- Link to tg
- Link to twitter

## Installation

Download [here](https://storage.com/release/v${VERSION}/app.tar.gz).

## Changelog

${CHANGELOG}
```