Gateway registration

A Gateway can create Acquirers, Payment Facilitators, and Merchants.

Endpoints

Method	Path	Description	Who can invoke it
`POST`	`/onboarding/gateways`	Register a gateway.	Admin.
`GET`	`/onboarding/gateways/{code}`	Retrieve a gateway.	Admin, the Gateway itself.
`PUT`	`/onboarding/gateways/{code}`	Update a gateway.	Admin.

Gateway OAuth credentials are always generated and required.

The client_id and client_secret are displayed only once during registration. Store them securely.

Review the operations for registering and managing Acquirers.