Make file input optional for recording methods in platform interface. Update docs.

Author: BeMacized

packages/camera/camera_platform_interface/lib/src/platform_interface/camera_platform.dart

@@ -67,8 +67,9 @@ abstract class CameraPlatform extends PlatformInterface {
     throw UnimplementedError('onCameraError() is not implemented.');
   }
 
-  /// Captures an image and returns the file where it was saved
-  Future<XFile> takePicture(int cameraId) {
+  /// Captures an image and returns the file where it was saved.
+  /// If no [file] parameter is provided, the file returned will be in a temporary location.
+  Future<XFile> takePicture(int cameraId, { XFile file }) {
     throw UnimplementedError('takePicture() is not implemented.');
   }
 
@@ -79,10 +80,12 @@ abstract class CameraPlatform extends PlatformInterface {
 
   /// Starts a video recording
   ///
+  /// If no [file] parameter is provided, the recording will be saved to a new file in a temporary location.
+  /// 
   /// The file is written on the flight as the video is being recorded.
-  /// If a file already exists at the provided path an error will be thrown.
+  /// If a file already exists at the path for the provided file instance, an error will be thrown.
   /// The file can be read as soon as [stopVideoRecording] returns it.
-  Future<void> startVideoRecording(int cameraId) {
+  Future<XFile> startVideoRecording(int cameraId, { XFile file }) {
     throw UnimplementedError('startVideoRecording() is not implemented.');
   }