TypeSpec extension should provide the ability to import the existing TypeSpecs for predefined scenarios

### Clear and concise description of the problem

In the TypeSpec playground, [typespec playground](https://typespec.io/playground/), we could import some typespecs with predefined scenarios, like Rest Framework, HTTP service etc.
Those spec would be a good point for user to start with instead of initializing project and writing typespec from empty tsp file/generic API only.
We shall also provide this functionality to import existing tsp file(s) in typespec extension to improve the authoring experience.

The existing scenarios in the playground:

API Versioning.
Rest framework,
Http service
Discriminated Unions.

There are some other tsp files for Azure and other protocols which I suggest those are lower priority.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.