Provide public api to allow FlutterEngine related context to be destoryed (flutter/engine#7610)

Provide public api to allow FlutterEngine related context  to be destoryed.

engine/src/flutter/shell/platform/darwin/ios/framework/Headers/FlutterEngine.h

@@ -50,6 +50,10 @@ FLUTTER_EXPORT
  * A newly initialized engine will not run the `FlutterDartProject` until either
  * `-runWithEntrypoint:` or `-runWithEntrypoint:libraryURI:` is called.
  *
+ * FlutterEngine created with this method will have allowHeadlessExecution set to `YES`.
+ * This means that the engine will continue to run regardless of whether a `FlutterViewController`
+ * is attached to it or not, until `-destroyContext:` is called or the process finishes.
+ *
  * @param labelPrefix The label prefix used to identify threads for this instance. Should
  *   be unique across FlutterEngine instances, and is used in instrumentation to label
  *   the threads used by this FlutterEngine.
@@ -119,6 +123,16 @@ FLUTTER_EXPORT
  */
 - (BOOL)runWithEntrypoint:(NSString*)entrypoint libraryURI:(NSString*)uri;
 
+/**
+ * Destroy running context for an engine.
+ *
+ * This method can be used to force the FlutterEngine object to release all resources.
+ * After sending this message, the object will be in an unusable state until it is deallocated.
+ * Accessing properties or sending messages to it will result in undefined behavior or runtime
+ * errors.
+ */
+- (void)destroyContext;
+
 /**
  * Sets the `FlutterViewController` for this instance.  The FlutterEngine must be
  * running (e.g. a successful call to `-runWithEntrypoint:` or `-runWithEntrypoint:libraryURI`)

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -149,13 +149,16 @@
 
 - (void)notifyViewControllerDeallocated {
   if (!_allowHeadlessExecution) {
-    [self resetChannels];
-
-    _shell.reset();
-    _threadHost.Reset();
+    [self destroyContext];
   }
 }
 
+- (void)destroyContext {
+  [self resetChannels];
+  _shell.reset();
+  _threadHost.Reset();
+}
+
 - (FlutterViewController*)viewController {
   if (!_viewController) {
     return nil;