Single-agent fields on phone number configuration

The single-agent fields on phone number configuration are deprecated in favor of the weighted *_agents lists.

Impacted API endpoints:

Create Phone Number, Import Phone Number, Update Phone Number, Get Phone Number, List Phone Numbers - fields: inbound_agent_id, inbound_agent_version, outbound_agent_id, outbound_agent_version, inbound_sms_agent_id, inbound_sms_agent_version, outbound_sms_agent_id, outbound_sms_agent_version