Add support for hosted services in .NET MAUI apps

Look into using [hosted services](https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.hosting.ihostedservice?view=dotnet-plat-ext-5.0). This would replace the icky `IMauiInitializeService` that was added in https://github.com/dotnet/maui/pull/2137, and also to enable users to use hosted services. MAUI will automatically call StartAsync/StopAsync at the appropriate time. Sadly, this pattern is async-only, whereas in MAUI the call sites are always (or at least almost always) sync. But given that this is app startup/shutdown, doing async-over-sync might be OK (we would block).

This is a follow-up to PR https://github.com/dotnet/maui/pull/2137.

Other notes:
* Need to consider this when implementing: https://github.com/dotnet/aspnetcore/pull/36122

Start by reviewing the IMauiInitializeService introduced in PR #2137, then read IHostedService and the linked ASP.NET Core PR. Done means MAUI supports hosted services and invokes StartAsync and StopAsync at the appropriate application lifecycle points while addressing the async-over-sync startup and shutdown concern.

Written by the indexing model from the issue text.