Add customer to phone thread
Add customer to phone thread

This is required. We're working with IDs for now, where we might want to
support the id or e-mail later.

From the docs:

> Customer adding the phone thread. Must contain a valid customer id or
> an email address. If the id field is defined, the email will be
> ignored. If the id is not defined, email is used to look up a
> customer. If a customer with the email in question does not exist a
> new one will be created, using all the optional fields and email.

> If a customer is matched either via id or email field, the rest of the
> optional fields is ignored.

Springest/help_scout,Springest/help_scout