[.NET] Optimize support for app extensions

We currently treat app extensions like separate apps, which means there are a lot of duplication.

This will be fixed by:

* [ ] Deduplication of runtime native libraries (libmono, libxamarin). This requires having these as frameworks, putting them in the main app only and having a rpath=../../Frameworks in the extension's native executable.
* Blocked for libmono because we don't have libmono as a framework yet (https://github.com/dotnet/runtime/issues/42846). We can still do this for libxamarin though.
* [ ] Deduplication of AOT-ed assemblies. This requires building assemblies to frameworks.
* [ ] Deduplication of linked output. This requires running the linker only once for all extensions + the main app, with all the assemblies in one big closure.
* [x] Deduplicate user frameworks between the app extensions and the app (tracked here: #17876).
* [x] Deduplicate icudt.dat between app extensions and the app (tracked here: #17877)