Ensure Hot Reload works

Hot Reload currently doesn't work for user assemblies that we modify during the build.

This typically happens at least for device builds (the registrar).

We need to avoid assembly modification for user assemblies when hot reload is enabled.

Work that needs to complete first:

* [x] https://github.com/dotnet/macios/issues/17693
* [x] In particular a new registrar mode using the type map feature will be required.
* [ ] Do something about the PreserveSmartEnumConversions step: https://github.com/dotnet/macios/blob/a92fbe4b82814678d90847303735fc1df41f1863/tools/dotnet-linker/PreserveSmartEnumConversionsStep.cs#L37-L38

Then:

* [ ] Verify that a default debug build for device doesn't modify any user assemblies.
* [ ] Modifying BCL/NuGet assemblies (assemblies not build using a project / without source code) is OK.
* [ ] If we do modify user assemblies, figure out a solution.

References:

* https://teams.microsoft.com/l/chat/19:e787a7cb1c964eaf948dd1bd7b2a5838@thread.v2/conversations?context=%7B%22contextType%22%3A%22chat%22%7D
* https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2260338
* https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2735766/?view=edit