[WindowsPlatform] Add fallback API for Windows XP.
[MacPlatform] Migrate MacKeychainPasswordProvider and add new API

Implement the new API for AddWebUserNameAndPassword and
GetWebUserNameAndPassword for IPasswordService changes, and
implement the Keychain API for looking this up.
[Core] Simplify RequestHelper a bit.
[WindowsPlatform] Correctly state the type of credential we need
[WindowsPlatform] Implement a password storage provider for Windows