Adds JWT validation for Apple and Google

Allows for simple validation of Sign in with Apple or google. In your routes, you can now do this:

public func registerFromSignInWithApple(req: Request) throws -> EventLoopFuture<User> {
    req.jwt.apple
        .verify(applicationIdentifier: "com.whoever.myapp")
        .map { (token: AppleIdentityToken) in
            let uniqueUserID = token.subject.value

            // Create the user in the database

            return user
    }
}