Provide custom marshaller for `USER_INFO_1`

[AaronRobinsonMSFT]

In PR #81480, the following P/Invoke was failing to compile when targeting .NET 8.

runtime/src/libraries/Common/tests/TestUtilities/System/WindowsIdentityFixture.cs

Lines 121 to 122 in 1671125

	[LibraryImport("netapi32.dll", SetLastError = true)]
	internal static partial uint NetUserAdd([MarshalAs(UnmanagedType.LPWStr)] string servername, uint level, ref USER_INFO_1 buf, out uint parm_err);

The failure is because in the current dotnet/runtime official build this P/Invoke is only compiled to target .NET 6 and so the LibraryImport source generated degrades to a DllImport. We should provide a custom marshaller for the struct below

runtime/src/libraries/Common/tests/TestUtilities/System/WindowsIdentityFixture.cs

Lines 127 to 138 in 1671125

	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
	internal struct USER_INFO_1
	{
	public string usri1_name;
	public string usri1_password;
	public uint usri1_password_age;
	public uint usri1_priv;
	public string usri1_home_dir;
	public string usri1_comment;
	public uint usri1_flags;
	public string usri1_script_path;
	}

This scenario could be made to "just work" with the following issue addressed: #81656

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Issue Details

---

In PR #81480, the following P/Invoke was failing to compile when targeting .NET 8.

runtime/src/libraries/Common/tests/TestUtilities/System/WindowsIdentityFixture.cs

Lines 121 to 122 in 1671125

	[LibraryImport("netapi32.dll", SetLastError = true)]
	internal static partial uint NetUserAdd([MarshalAs(UnmanagedType.LPWStr)] string servername, uint level, ref USER_INFO_1 buf, out uint parm_err);

The failure is because in the current dotnet/runtime official build this P/Invoke is only compiled to target .NET 6 and so the LibraryImport source generated degrades to a DllImport. We should provide a custom marshaller for the struct below

runtime/src/libraries/Common/tests/TestUtilities/System/WindowsIdentityFixture.cs

Lines 127 to 138 in 1671125

	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
	internal struct USER_INFO_1
	{
	public string usri1_name;
	public string usri1_password;
	public uint usri1_password_age;
	public uint usri1_priv;
	public string usri1_home_dir;
	public string usri1_comment;
	public uint usri1_flags;
	public string usri1_script_path;
	}

This scenario could be made to "just work" with the following issue addressed: #81656

Author:	AaronRobinsonMSFT
Assignees:	-
Labels:	area-System.Runtime.InteropServices
Milestone:	-

[elinor-fung]

For anyone that might want to pick this up, here is an example of a struct with custom marshalling used in downlevel and current targets:

runtime/src/libraries/System.Diagnostics.EventLog/src/System/Diagnostics/Reader/UnsafeNativeMethods.cs

Lines 344 to 348 in 876a97d

	#if NET7_0_OR_GREATER
	[NativeMarshalling(typeof(Marshaller))]
	#endif
	[StructLayout(LayoutKind.Sequential)]
	internal struct EvtRpcLogin