[Mono] Fix support for nested structs with explicit layout (#61467)

[Mono] Fix support for nested structs with explicit layout (#61467)

* Add test case for nested struct with explicit layout

* Add license

* Modify test

* Temporarily enable the test in the CI pipeline

* Allow running Mac Catalyst builds in App Sandbox

* Allow enabling app sandbox for the other Mac Catalyst sample

* Revert "Allow enabling app sandbox for the other Mac Catalyst sample"

This reverts commit 8c3c9f92daa47d1de844dbe897155d119415fb74.

* Revert "Allow running Mac Catalyst builds in App Sandbox"

This reverts commit 1e404165d1859bc9e25dbcdb0b4dc74a839dc708.

* Add more test cases

* WIP: First implementation of the recursive check

* Improve code

* Fix test case

* Add more test cases

* Unify test cases

* Bugfixes

* Remove test which behaves differently depending on the target platform

* Remove sequential layout test which behaves differently on Linux and on Windows

* Reorganize test cases

* Remove call to mono_class_setup_fields

* Fix embedding detection

* Fix layout validation for generic structs

* Remove unintentional change

* Revert temporary change to the runtime.yml pipeline

* Code clean-up

* Revert unrelated change

* Use getters instead of directly accessing MonoClass fields