Add Identity Platform accounts API support

[vamsii777]

Description

Implements the accounts API portion of Google Identity Platform (Identity Toolkit)

Accounts API Implementation (/v1/accounts:*)

• [x] signInWithPassword - Sign in with email/password
• [x] signInWithCustomToken - Sign in with custom token
• [x] signInWithEmailLink - Sign in with email link
• [x] signInWithGameCenter - Sign in with Game Center
• [x] signInWithIdp - Sign in with Identity Provider
• [x] signInWithPhoneNumber - Sign in with phone number
• [x] sendVerificationCode - Send SMS verification code
• [x] sendOobCode - Send out-of-band confirmation code
• [x] createAuthUri - Check email registration
• [x] resetPassword - Reset account password
• [x] lookup - Get account information
• [x] delete - Delete user account
• [x] update - Update account information
• [x] verifyIosClient - Verify iOS client authenticity