Improve apphost discovery.

[mitchdenny]

This PR improves usability of the aspire [run|add|publish] commands by searching for valid app hosts in sub-directories, using the current working directory as a starting point. Here is a walkthrough video showing it in action. The important thing to note here is that we did not change into the src/HelloAspire.AppHost` sub directory in order for this command to work.

aspire-recursive-search.mp4

Note that this recursive search behavior can be expensive in projects with a large number of *.csproj files so it is possible to place a .aspire/settings.json file in the current directory or any parent directory to short-circuit the discovery process and point directly to a well known apphost.

Included in this PR is a mechanism whereby this configuration file is automatically dropped in the <workingdir>/.aspire/settings.json path to avoid subsequent invocations taking as long. The idea is that by the time it becomes a problem in a repository the settings file is present.

[davidfowl]

Is there a fast path for csproj in the current directory or do I pay for this everytime?

[mitchdenny]

If you invoke aspire run in the app host directory we will drop a .aspire/settings.json file so that subsequent executions don't traverse. In effect if you are in the apphost folder you'll only see one csproj file anyway, so it'll be almost the same as the current behavior.