[Trimming] Simpler image source service provider

[simonrozsival]

This is an attempt to fix trimming warnings in ImageSourceServiceProvider without changing the existing APIs. The main idea of this approach is to iterate over all the registered image source to image source service mappings (there aren't that many usually + we're caching the result of the lookup anyway) and find the best match for the input image source type.

There are just a few TODOs:

• I added two new methods to IImageSourceServiceCollection and they're not part of the publicly declared API yet
• While all unit tests and most device tests on Mac Catalyst are passing, there are 55 failing device tests in scroll view tests that I'm having hard time diagnosing and fixing (ObjCException which hides the original exception).

The main question I think we need to ansewer: Is this a better way to go about the ImageSourceServiceProvider than what Mitch has been working on with keyed services? The change is certainly a lot less invasive, and it doesn't significantly change how DI works, but maybe we need the disruptive change to be able to phase out MauiFactory eventually?

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