Relax the protocol requirement for Snapshotting extension

[Kyle-Ye]

When wrapping a custom assert function for SwiftUI.View, we may want to write the following code.

#if canImport(AppKit)
typealias PlatformHostingController = NSHostingController
#else
typealias PlatformHostingController = UIHostingController
#endif

func viewAssertSnapshot<V: View, Format>(
    of value: @autoclosure () throws -> V,
    as snapshotting: Snapshotting<PlatformHostingController<V>, Format> = .image,
    ...
) {
    assertSnapshot(
        of: PlatformHostingController(rootView: try value()),
        as: snapshotting,
        named: name,
        record: recording,
        timeout: timeout,
        fileID: fileID,
        file: filePath,
        testName: testName,
        line: line,
        column: column
    )
    ...
}

It does not compile now since image extension has a requirement of Value == UIViewController.

And the workaround is change Snapshotting<PlatformHostingController<V>, Format> to Snapshotting<UIViewController, Format>.

I think it is common we define some UIViewController's subclass and use the new subclass as the generic. So I propose we relax the requirement for the common UI extension of Snapshotting.