AppHost search should run in parallel.

[mitchdenny]

At the moment when executing aspire run, aspire add or aspire publish in a source tree results in a search for all *.csproj files which are an apphost. For each project it runs dotnet msbuild to fetch information to determine whether the project is an app host and then stores this information to reduce future lookup time.

We should consider a few options for optimizing this experience. A few options:

1. Run all dotnet msbuild calls in parallel (or in parallel batches)
2. Prompt for the user to select their csproj if over a certain number of csproj files are found.