Expose the version / tag that will be generated in the PR json

Can you consider outputting the release version info that the PR will create in the `pr json`.

This opens up the opportunity to do any kind of extra file manipulation.. for example...

In a github action.. We can update the PR files with any additional infor we want for example...
```yaml
- uses: googleapis/release-please-action@v4
id: release
with:
token: "${{ secrets.GITHUB_TOKEN }}"
config-file: release-please-config.json
manifest-file: .release-please-manifest.json

## we have just created a PR now modify it however we want eg.. replace the image tag in a k8s manifest
- name: Modify pull-request files after RP
if: steps.release.outputs.prs_created == 'true'.
uses: jaywcjlove/github-action-modify-file-content@main
with:
branch: ${{ fromJson(steps.release.outputs.pr).headBranchName }}
body: "{{ fromJson(steps.release.outputs.pr).version }} .. **I didnt see it in json**
path: 'deployment.yaml'
```

Thanks