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

Video.js

@@ -418,6 +418,7 @@ Video.propTypes = {
     certificateUrl: PropTypes.string,
     getLicense: PropTypes.func,
   }),
+  localSourceEncryptionKeyScheme: PropTypes.string,
   minLoadRetryCount: PropTypes.number,
   maxBitRate: PropTypes.number,
   resizeMode: PropTypes.string,