[MonoAPI] Split type and function headers, add MONO_API_FUNCTION macro (#65446)

[MonoAPI] Split type and function headers, add MONO_API_FUNCTION macro (#65446)

* [monoapi] Split type and function headers, add MONO_API_FUNCTION macro

The idea is that the function header can be included multiple times with different definitions of MONO_API_FUNCTION in order to make it easier to re-used the definitions for embedding the runtime in late-binding scenarios

* rename public mono/mini subdirectory to mono/jit

  To match how embedders see the tree.

  Update the runtime to include <mono/jit/jit.h> instead of <mono/mini/jit.h>.

  No change in public API

* Add needed includes to mobile testing host templates

* add unstable API comment to the details headers, too

* use install(FILES) to copy the headers

  not other stray files to the include dir

* update CODEOWNERS

* Remove duplicate definitions of mono_event_get_remove_method

  This was already duplicated before the header reorganization, e.g. here https://github.com/dotnet/runtime/blob/b9a55b4f52243325359ced26e3d4b31ccacdc381/src/native/public/mono/metadata/class.h#L279-L282