Upsert merchant

Creates or updates a merchant based on the merchant's `reference_id`. If a merchant with a matching `reference_id` is found, it will be updated. If no matching merchant is found, a new one will be created. The HTTP response code (`200 OK` or `201 Created`) indicates whether the resource was updated or created, respectively. <Info> `merchant_id` is the primary unique identifier for all Merchant APIs. Be cautious when using `reference_id` as a unique identifier. Merchants must have an `address` or `site_url` set. </Info> **This endpoint is not rate limited.** Scopes: `MERCHANTS_WRITE`