Update `limits` of outgoing payment grant creation

[mkurapov]

Context

Existing PR to take over from @raducristianpopa: #499

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
• Update the client types to make sure that when creating an outgoing payment grant, it allows either debitAmount XOR receiveAmount. This is a breaking change for the client.