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

ios/Video/Features/RCTPlayerOperations.swift

@@ -1,5 +1,6 @@
 import AVFoundation
 import MediaAccessibility
+import Promises
 
 let RCTVideoUnset = -1
 
@@ -157,4 +158,23 @@ enum RCTPlayerOperations {
         }
         
     }
+
+    static func seek(player: AVPlayer, playerItem:AVPlayerItem, paused:Bool, seekTime:Float, seekTolerance:Float) -> Promise<Bool> {
+        let timeScale:Int = 1000
+        let cmSeekTime:CMTime = CMTimeMakeWithSeconds(Float64(seekTime), preferredTimescale: Int32(timeScale))
+        let current:CMTime = playerItem.currentTime()
+        let tolerance:CMTime = CMTimeMake(value: Int64(seekTolerance), timescale: Int32(timeScale))
+        
+        return Promise<Bool>(on: .global()) { fulfill, reject in
+            guard CMTimeCompare(current, cmSeekTime) != 0 else {
+                reject(NSError())
+                return
+            }
+            if !paused { player.pause() }
+
+            player.seek(to: cmSeekTime, toleranceBefore:tolerance, toleranceAfter:tolerance, completionHandler:{ (finished:Bool) in
+                fulfill(finished)
+            })
+        }
+    }
 }