Improved Defer Fulfillment (#5127)

* improved defer fullfillment

* applied also for async streams

* fix xcodeproject

* better documentation

* restict deferFulfillment  only for Publisher which can never fail

UnitTests/SupportingFiles/target.yml

@@ -52,6 +52,7 @@ targets:
     - path: ../../DevelopmentAssets
     - path: ../../ElementX/Sources/Other/Extensions/Publisher.swift
     - path: ../../ElementX/Sources/Other/Extensions/XCTestCase.swift
+    - path: ../../ElementX/Sources/Other/DeferredFulfillment.swift
     - path: ../../ElementX/Sources/Other/InfoPlistReader.swift
     - path: ../../Tools/Scripts/Templates/SimpleScreenExample/Tests/Unit