Add samples for different patterns of composing services from multiple repositories into an AppHost project

From https://github.com/dotnet/aspire/issues/829

We should investigate how we can add some samples of various patterns that could be used to orchestrate projects and services from multiple repos rather than mono-repo. It's common to split up service ownership by team or simply to put them in their own repos.

Some patterns I imagine might emerge:

• git sub-modules for service repos and a parent repo with the .sln file and AppHost project, if you want to build and run all services locally
• Services published as container images that are then composed into an AppHost project as containers, if you don't want to build the services locally but you do want to run them locally
• Dev-instances of services deployed to dedicated environments and/or locally that are then composed into an AppHost project as https://github.com/dotnet/aspire/issues/775, if you don't want to build or run the services locally

Start with the linked Aspire issue #829 and the AppHost project context. Compare the three proposed patterns: git submodules, composed container images, and deployed or local dev instances. Done means adding representative samples that demonstrate the selected multi-repository composition patterns.