Enable Discovery of 3rd Party Integrations and Templates in Aspire

### Is there an existing issue for this?

- [x] I have searched the existing issues

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

Today aspire's add command is fixed to the set of known packages (the community toolkit and aspire integrations). We want to allow users to opt in to extra integrations showing up in this experience. There are 2 scenarios:

- A company wants to light up 1st party integrations and starter templates.
- We want to enable discovering extra community packages as part of this experience.

### Describe the solution you'd like

Add support for discovering 3rd party integrations and templates. This would allow:

- Companies to surface 1st party integrations and starter templates, making them available to users in Aspire's add command experience.
- Users to discover and opt into extra community packages, expanding the set of available integrations beyond the built-in toolkit and official Aspire integrations.

Alternative solutions considered: Maintain a curated list of integrations manually, but this limits extensibility and community participation.

### Additional context

This feature should include mechanisms for:
- Registering new integrations and templates so they show up for users.
- Opt-in UI/CLI experience for surfacing extra integrations.
- Ensuring both 1st party and community packages can be discovered without compromising security or quality.
- Documentation for how companies and community contributors can register their packages/templates.

Related areas: extensibility, acquisition, templates, integrations.