Using generic updater for a single file that contains multiple component's versions

Related to #1857

**Is your feature request related to a problem? Please describe.**

I have a helm `values.yaml` which contains versions for multiple apps (because I have this master app that spawns workers).

From https://github.com/googleapis/release-please/issues/1857#issuecomment-4032948933

> Personally I want this feature to be able to do this:
>
> ```yaml
> myapps:
> app1:
> version: 1.2.3 # x-release-please-app1-version
> config: {}
> app2:
> version: 6.4.3 # x-release-please-app2-version
> app3: ...
> ```
>
> So I guess I would put in release-please-config:
>
> ```json
> {
> "type": "generic",
> "path": "/helm/mychart/values.yaml"
> "inline-update-regex": "x-release-please-app1-version"
> }
> ```
>
> Notes:
> * I have to use the generic updater because the yaml one removes comments (#2195)
> * This use-case would also be covered by release-please searching for `x-release-please-version-${package_name}`, without any extra config.

**Describe the solution you'd like**

Either be able to pass `inline-update-regex` or release-please, by default, should search for `x-release-please-myapp-version`.