VEX-5938: Update resource loader to handle encrypted local files (#12)

Adds offline decryption key and uses it to decrypt content during offline playback

Jira: VEX-5938
https://jira.tenkasu.net/browse/VEX-5938

- Update to accept scheme for key required to play offline playback
- Uses provided scheme to intercept call from player and return the key
- Fixes player item observer removal pattern

### Reviews
- Major reviewer (domain expert): @armadilio3

react-native-video.podspec

@@ -12,11 +12,12 @@ Pod::Spec.new do |s|
   s.homepage       = 'https://github.com/react-native-community/react-native-video'
   s.source       = { :git => "https://github.com/react-native-community/react-native-video.git", :tag => "#{s.version}" }
 
-  s.ios.deployment_target = "8.0"
+  s.ios.deployment_target = "9.0"
   s.tvos.deployment_target = "9.0"
 
   s.subspec "Video" do |ss|
     ss.source_files  = "ios/Video/**/*.{h,m,swift}"
+    ss.dependency "PromisesSwift"
   end
 
   s.subspec "VideoCaching" do |ss|