Update `limits` of outgoing payment grant creation

[mkurapov]

Context

Currently, when we create an outgoing payment grant request, we can pass in a limits attribute with debitAmount and/or receiveAmount. For simplicity of the API as well as the simplicity of documenting how these amounts are used, we should update the spec to allow only setting limits.debitAmount OR limits.receiveAmount, not both.

Todos

• [ ] Update the auth-server spec to allow debitAmount or receiveAmount in the limits attribute of outgoing payment grant creation