[Feature]: make the Notifications package linker-friendly

[Sergio0694]

Describe the problem

Currently, the Microsoft.Toolkit.Uwp.Notifications package is heavily relying on reflection for the XML serialization of the various toast/tile/visual templates it can create. This fundamentally breaks trimming, and can result in incorrect behavior (ie. broken notifications) on anyone enabling it (eg. on UWP). The workaround is to explicitly add a runtime directive to preserve public members from this assembly, but that's (1) brittle and (2) cumbersome, especially for beginners. It's also just not ideal, as doing so still ends up preserving more stuff than what would actually be needed, hence increasing the package size.

Describe the solution

Since all the types being serialized are well known and internal, there is no need to actually use reflection.
As such, the code should be refactored to not use it anymore, making it linker-friendly.

Alternatives

Do nothing, and keep using reflection. Meh.

Additional info

No response

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Yes, I'd like to be assigned to work on this item.

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