Merchant registration

Merchants occupy the last level in the specified entity hierarchy.

Endpoints

Method	Path	Description	Who can invoke it
`POST`	`/onboarding/merchants`	Register a merchant.	Admin, Gateway, Acquirer, PF.
`GET`	`/onboarding/merchants/{code}`	Retrieve a merchant.	Admin, Gateway, Acquirer, PF.
`PUT`	`/onboarding/merchants/{code}`	Update a merchant.	Admin, Gateway, Acquirer, PF.

OAuth credential generation is configurable with generate_credentials: "true" or "false".

When generated, the client_id and client_secret are displayed only once during registration. Store them securely.

Review how registered data becomes available to the authorization flow.