Add support for sending magic links to inexistent accounts

[hichamboushaba]

Description

Part of woocommerce/woocommerce-ios#14419

This PR updates the magic link request with an option to pass the option create_account, this option allows to use the same request for signup flow needed for the above task.

Testing Details

Use woocommerce/woocommerce-ios#14431

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[hafizrahman]

First time reviewing Obj C so bear with me if anything's missing. But from what I can tell the logic is pretty sound here.