Add multi solutions support

### 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.

Hello,

I am trying to use Aspire using 4 existing solutions. Each of these solutions contain between 10+ and 60+ projects.

I managed to create an Aspire host project referencing the 4 "main" projects of each solution and this works very well.
However when using the Aspire solution not all projects are there and therefore it is not possible to easily navigate code or rebuild everything.
I could manually add all existing projects into the Aspire solution but with the high number of projects this would be quite long plus it would require maintenance when we add new projects in the existing 4 solutions.

### Describe the solution you'd like

I am not fully sure if this should be managed by Aspire or Visual Studio but in this scenario being able to reference solutions from a solution could greatly help.

Thanks very much.

### Additional context

_No response_