HeadContent Append Mode

### Is there an existing issue for this?

- [X] I have searched the existing issues

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

This was already requested and closed "by design"
https://github.com/dotnet/aspnetcore/issues/51864

For component authoring, it would be ideally to have components that can add to the HeadOutlet individually, specially for adding script tags,for example, GoogleTagManager component.

There's also the problem that if you use 2 HeadOutlets, there's no error at all, only one gets rendered and the other is discarded silently. poor developer experience.

### Describe the solution you'd like

Having 2 HeadContent override each other throws and error.
Opt in into an `Optional` parameter so it doesn't throws and works as today.
Opt in into an `Append` parameter and it appends content to the already existing (or not) HeadOutlet.

### Additional context

_No response_