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

README.md

@@ -317,6 +317,7 @@ var styles = StyleSheet.create({
 * [trackId](#trackId)
 * [useTextureView](#usetextureview)
 * [volume](#volume)
+* [localSourceEncryptionKeyScheme](#localSourceEncryptionKeyScheme)
 
 ### Event props
 * [onAudioBecomingNoisy](#onaudiobecomingnoisy)
@@ -915,6 +916,18 @@ Adjust the volume.
 
 Platforms: all
 
+#### localSourceEncryptionKeyScheme
+Set the url scheme for stream encryption key for local assets
+
+Type: String
+
+Example:
+```
+localSourceEncryptionKeyScheme="my-offline-key"
+```
+
+Platforms: iOS
+
 
 ### Event props