This endpoint will return the customer preference tags stored in Rejoiner.
You can use either the Rejoiner Customer ID or the customer's email address.
GET /customers/preference_tags/CUSTOMER_ID/
GET /customers/preference_tags/by_email/?email=CUSTOMER_EMAIL
{
"id": "foobar",
"email": "[email protected]",
"preference_tags": ["one-per-week"]
}