Install GHCP App canvases with `aspire agent init`

## Problem

`aspire agent init` currently supports installing skills and MCPs. The [`aspire-skills`](https://github.com/microsoft/aspire-skills) repository now also includes canvases, such as the [Aspire Doctor canvas](https://github.com/microsoft/aspire-skills/tree/main/extensions/aspire-doctor), but the Aspire CLI has no way to distribute them.

## Proposed solution

Update `aspire agent init` to support additional agent asset types.

Users should be able to select extensions similarly to skills, with only assets supported by their detected agent being offered. Skills and extensions are separate bundles, but the implementation should reuse shared bundle infrastructure where appropriate.