Multiple bicep extensions that share an implementation

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

As part of [Radius](https://github.com/radius-project/radius) we're adding user-defined types **resource types**. This has an overlapping name with the UDT feature of Bicep, but it's different :laughing:.

This will allow a user to:

- Define their own resource types using a simplified YAML format
- Upload that data to Radius so our API can process it
- Generate a Bicep extension for local tooling support

This is a lot like Kubernetes CRDs but inside Radius.

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The challenge is that there is no "server side" extension in the deployment engine. We need to instead treat these extensions as part of Radius, and let them get processed by the Radius extension in the deployment engine.

Essentially, users are bringing their own type definitions that augment our extension, and can be processed by our extension.

This is extensible, so we can't hardcode names.

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As a thought experiment, the Kubernetes extension in Bicep has the same challenge. It only knows about resource types that are *in the box*, yet Kubernetes is extensible. Any user that wanted to ship a *library* of Kubernetes types would end up with the same requirements.

**Describe the solution you'd like**

I think the ideal thing would be an "alias" feature. A way for a client-side extension to instruct the server which extension id should be responsible for processing.

Any solution would be acceptable as long as it...

- Is extensible by users (no hardcoded names)
- Doesn't require users to implement server-side Bicep extensions

We have control over *our* deployment engine code as well, but we'd like to rely on extensibility where possible.

Start by reading the deployment engine's extension model and how the client-side and server-side Bicep extensions interact. Compare the Radius resource-type requirement with the Kubernetes extension example, then define an extensible way for users to route types to an existing extension without implementing server-side extensions. Done means the approach supports user-defined names and can be used by Radius.

Written by the indexing model from the issue text.