[Feature] Need access to control templating/retemplating from Reactor

### Problem

When I want to completely retemplate a control I think I'd prefer doing that with a Xaml resource style, but then there needs to be an easy way to apply it

### Proposed solution

Unsure of the design, this is more of an exploration

### Alternatives considered

_No response_

### Additional context

_No response_

### Confirmation

- [x] I have searched [existing issues and specs](https://github.com/microsoft/microsoft-ui-reactor/issues?q=is%3Aissue) for prior discussion of this idea.

Start by reviewing how Reactor currently exposes control templating and how WinUI XAML resource styles apply templates. Define the API for applying a resource style and the expected retemplating behavior, then validate the proposed design against a representative control. Done means the design is agreed and the control-template lifecycle is clearly specified.

Written by the indexing model from the issue text.