Update the template screen to use the new(ish) Observation framework. (#4077)

* Update the template screen to use the new(ish) Observation framework.

* Add a variant of deferFulfillment that supports observables.

* Update snapshot fulfilment to work with either a publisher or a stream.

ElementX/Sources/Other/Extensions/AsyncSequence.swift

@@ -9,4 +9,19 @@ extension AsyncSequence {
     func first() async rethrows -> Self.Element? {
         try await first { _ in true }
     }
+    
+    /// Type-erases the sequence into a newly constructed asynchronous stream. This is useful until
+    /// we drop support for iOS 17, at which point we can replace this with `any AsyncSequence`.
+    @available(iOS, deprecated: 18.0, message: "Use `any AsyncSequence` instead.")
+    func eraseToStream() -> AsyncStream<Element> {
+        var asyncIterator = makeAsyncIterator()
+        return AsyncStream<Element> {
+            do {
+                return try await asyncIterator.next()
+            } catch {
+                MXLog.warning("Stopping stream: \(error)")
+                return nil
+            }
+        }
+    }
 }