Update string marshallers to V2 shape (#71849)

Update string marshallers to V2 shape (#71849)

* Update to string marshallers to new V2 shape.

* Fix incorrect BStr casing.

* Use NativeMemory.Alloc/Free where possible.
Avoid initialization of marshaller when not used.
Remove unneccessary unmanaged->managed APIs for In marshallers.

* Add a ClassLoader::LoadTypeByNameThrowing overload
  that can be used to load nested classes.

* Allocate precise amounts for UTF8 and ANSI marshallers.

* Use CoTaskMemAlloc/Free

* Keep allocation in local.