Payment Facilitator registration

A Payment Facilitator can create Merchants.

Endpoints

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

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 the operations for registering and managing Merchants.