Implement code signing for client ios SDK (#974)

* Add JWT Library to ios project

* Add new option PublicKey to Info.plist

* Implement code signing mechanism

Update JWT library to 3.0.0-beta4
Implement signature verification mechanism

* Undo redundant changes

* Minor improvements

* Minor fixes

Add additional checking for specific update situations
Fix bugs
Refactor method names

* Add new dependencies to podspec

* Minor improvements

Fix log messages

Author: ruslan-bikkinin

CodePush.podspec

@@ -28,4 +28,13 @@ Pod::Spec.new do |s|
     ss.private_header_files = 'ios/CodePush/SSZipArchive/*.h', 'ios/CodePush/SSZipArchive/aes/*.h', 'ios/CodePush/SSZipArchive/minizip/*.h'
   end
 
+  s.subspec 'JWT' do |jwt|
+    jwt.source_files = 'ios/CodePush/JWT/**/*.{h,m}'
+    jwt.private_header_files = 'ios/CodePush/JWT/**/*.h'
+  end
+
+  s.subspec 'Base64' do |base64|
+    base64.source_files = 'ios/CodePush/Base64/**/*.{h,m}'
+    base64.private_header_files = 'ios/CodePush/Base64/**/*.h'
+  end
 end