Expose ArrayMarshaller and PointerArrayMarshaller

[elinor-fung]

Resolves #66623

This just moves the types, renames PtrArrayMarshaller -> PointerArrayMarshaller, and adds doc. I didn't change the actual implementation of the marshallers or how they are used.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Tagging subscribers to this area: @dotnet/interop-contrib
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Resolves #66623

This just moves the types, renames PtrArrayMarshaller -> PointerArrayMarshaller, and adds doc. I didn't change the actual implementation remains the same.

Author:	elinor-fung
Assignees:	-
Labels:	area-System.Runtime.InteropServices
Milestone:	7.0.0

[jkoritzinsky]

We should also remove this code block now that we're exposing the types:

runtime/src/libraries/System.Runtime.InteropServices/gen/LibraryImportGenerator/PInvokeStubCodeGenerator.cs

Lines 201 to 209 in 3b6b812

	// TODO: Remove once helper types (like ArrayMarshaller) are part of the runtime
	// This check is to help with enabling the source generator for runtime libraries without making each
	// library directly reference System.Memory and System.Runtime.CompilerServices.Unsafe unless it needs to
	if (p.MarshallingAttributeInfo is MissingSupportMarshallingInfo
	&& (environment.TargetFramework == TargetFramework.Net && environment.TargetFrameworkVersion.Major >= 7))
	{
	throw new MarshallingNotSupportedException(p, this);
	}

[jkoritzinsky]

LGTM with my one comment

[elinor-fung]

Failures were:

• mono minijit x64 test failure: loopinfinally_do.dll exception #68187
• Windows arm64 Tests failing on DDARM64-132 #67821 / DDARM64-125 fails all jobs arcade#9088