Add implementation of Aspire service

[tmat]

Adds Aspire.Tools.Service source package that contains code shared between WebTools (VS/VSCode) and dotnet-watch.

TODO: will follow up with moving some tests

[tmat]

@BillHiebert @DamianEdwards @phil-allen-msft

[BillHiebert]

Should the namespace change? Specifically, the WebTools part.

[tmat]

@BillHiebert Doesn't matter too much since all the types are internal, but if you want it to change, we can change it.

Microsoft.Aspire?

We have Microsoft.AspNetCore.Watch.BrowserRefresh namespace for browser refresh component, dotnet-watch uses Microsoft.DotNet.Watcher.

I suggest we keep it as is for now and once we consume the shared package from WebTools repo we can change the namespace.