Update AS spec to ensure correct limits during outgoing payment grant creation

[mkurapov]

Context

• From prerequisite Update limits of outgoing payment grant creation open-payments#471:

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.

When this change lands, we need to update the open-payments client in the AS to make sure we are correctly validating the incoming request.

Todos

• Update open-payments client in AS after issue above is finished