`UnsafeAccessorTypeAttribute` support for field accessors

The initial proposal in #90081 mentioned that the inaccessible type problem applies to both fields and methods, but the implementation skipped support for fields due to type safety issues: https://github.com/dotnet/runtime/pull/114881#issuecomment-2856649342 - having a `ref object` return type for the accessor isn't memory safe, it would need `TypedReference` support for that.

Needing it for fields is probably less common, but I just ran into a situation where I need it for .NET 10 (as a replacement for `HttpContent.TryGetBuffer` that's in older versions):
```cs
[UnsafeAccessor(UnsafeAccessorKind.Field)]
[return: UnsafeAccessorType("System.Net.Http.HttpContent+LimitArrayPoolWriteStream, System.Net.Http")]
private static extern ref object? _bufferedContent(HttpContent content);
```
This ends up throwing `Invalid usage of UnsafeAccessorTypeAttribute`.