[layers v2] Extend `azd add` for extension-contributed items

Part of #9765.

Let extensions contribute their own addable items to `azd add`, alongside azd's built-in catalog (databases, storage, AVM, etc.), instead of extensions building their own separate `add`-like command.

- Needs a new extension API for registering, discovering, and invoking a contribution — azd owns validation, layer assignment, and applying the resulting project mutation; the extension owns its own domain-specific prompts/config.
- Open UX questions to resolve: what does the `azd add` flow look like when the contributing extension isn't installed yet — can we show/preview the item before install, and auto-install it the way azd already does for something like `azd ai`?
- Open question on the contribution model shape and how it's displayed: is it hierarchical (e.g., categories like "Azure Foundry" → extension-provided sub-items) or flat? Is there another representation that works?

One thing that comes out of this work is making the `azd add` list dynamic, in general, so we can easily search through our template catalog, or through the list of existing AVMs.