swift: add some missing ResponseFilter functions (#1000)

These are specified in the Kotlin interface, but not in Swift. They'll be wired up separately.

Signed-off-by: Michael Rebello <me@michaelrebello.com>
Signed-off-by: JP Simard <jp@jpsim.com>

mobile/examples/swift/hello_world/DemoFilter.swift

@@ -22,4 +22,8 @@ struct DemoFilter: ResponseFilter {
   func onResponseTrailers(_ trailers: ResponseTrailers) -> FilterTrailersStatus<ResponseTrailers> {
     return .continue(trailers)
   }
+
+  func onError(_ error: EnvoyError) {}
+
+  func onCancel() {}
 }

mobile/library/swift/src/filters/ResponseFilter.swift

@@ -37,4 +37,18 @@ public protocol ResponseFilter: Filter {
   ///
   /// - returns: The trailer status containing body with which to continue or buffer.
   func onResponseTrailers(_ trailers: ResponseTrailers) -> FilterTrailersStatus<ResponseTrailers>
+
+  /// Called at most once when an error within Envoy occurs.
+  ///
+  /// This should be considered a terminal state, and invalidates any previous attempts to
+  /// `stopIteration{...}`.
+  ///
+  /// - parameter error: The error that occurred within Envoy.
+  func onError(_ error: EnvoyError)
+
+  /// Called at most once when the client cancels the stream.
+  ///
+  /// This should be considered a terminal state, and invalidates any previous attempts to
+  /// `stopIteration{...}`.
+  func onCancel()
 }