[meta] Replace the custom linker steps with different solutions

[rolfbjarne]

Note: this issue is incomplete, it still lacks a lot of detail.

We're using custom linker steps during our build in order to easily reuse pre-existing code we have for legacy Xamarin.iOS/Xamarin.Mac builds (mtouch/mmp).

• The ILLinker team wants to remove support for custom linker steps, since it simplifies their code base and makes it easier for them to innovate.
• The NativeAOT compiler trims code, but doesn't support custom linker steps. If we want to support NativeAOT without having to also run ILLinker first, we'll have to replace the custom linker steps with a different solution.

The custom linker steps are used for multiple purposes, and multiple different solutions will have to be implemented. The main idea is to add msbuild tasks that are executed before and after the trimmer (either ILLinker or NativeAOT).

• We need to track of build times. A big advantage of the custom linker steps is that assemblies are only loaded once, during trimming. Adding other msbuild tasks that are executed before and after trimming means we'll have to load assemblies more than once, and this might slow down the build. We'll have monitor this and get numbers how bad it is for each step of the way.

(add list of all the custom linker steps)
(separate the removal of each step into it's own issue)

For app extensions, we'll need to generate an XML with everything we want rooted (ref: #19037). This way we'll hopefully be able to avoid this workaround: #19049