How to implement a custom changelog

Hello 👋🏻

I'm trying, as mentioned in the [documentation](https://github.com/googleapis/release-please/blob/main/docs/customizing.md#adding-additional-changelog-types), to overwrite the default changelog generation.

I've followed the [default implementation in `src/changelog-notes/default.ts`](https://github.com/googleapis/release-please/blob/main/src/changelog-notes/default.ts)

`./changelog-types.ts`
*(at the root of my project, alongside release-please-config.json and .release-please-manifest.json)*
```ts
export class CustomChangelogNotes implements ChangelogNotes {
...
}
```

I'm confused about how actually letting release-please pick that file. Using `changelog-type` seems to be a good start but according to the schema, it should only be `default` or `github`

`release-please-config.json`
```json
{
"packages": {
".": {
"changelog-path": "CHANGELOG.md",
"release-type": "simple",
"draft": false,
"prerelease": false,
"changelog-type": "custom"
}
},
"$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json"
}
```

PS: I'm using Github Action, which takes [`changelog-notes-type`](https://github.com/google-github-actions/release-please-action/tree/v3) as input, but it should be the same as `changelog-type`.

Any idea? Thanks 🙏🏻