FR: pull from external config

As a developer, I would like to have a single, centralized configuration for all projects in my GH org that are using release-please as a GitHub action. There should also be an option to extend this configuration on a per-repo basis, in case certain adjustments need to be made.

[Renovate](https://github.com/renovatebot/renovate/) has a good [example](https://github.com/renovatebot/renovate/blob/main/docs/usage/config-presets.md) of how this is implemented in its service. Ideally, I could use an `extends` field in my `release-please-config.json`:
```json
{
"extends": "MyOrgName/.github",
"packages": { ".": {} },
"release-type": "node"
}
```

this would merge the individual repo's configuration with the configuration found in my organization's `.github` repo, overwriting the parent's configuration with the child's values.