Add RootSelectionSet convenience initializer from inline fragment (#748)

Author: AnthonyMDev

apollo-ios/Sources/ApolloAPI/SelectionSet.swift

@@ -128,8 +128,16 @@ extension SelectionSet where Fragments: FragmentContainer {
   public var fragments: Fragments { Fragments(_dataDict: __data) }
 }
 
+// MARK: - Root Entity Type Conversion Helpers
+
 extension InlineFragment {
   @inlinable public var asRootEntityType: RootEntityType {
     RootEntityType.init(_dataDict: __data)
   }
 }
+
+extension RootSelectionSet {
+  public init<T: InlineFragment>(_ inlineFragment: T) where T.RootEntityType == Self {
+    self = inlineFragment.asRootEntityType
+  }
+}