Update /deliveryservices/{{ID}}/servers/eligible to use data from new interfaces tables

[ocket8888]

I'm submitting a ...

• new feature / enhancement request

Description

The /deliveryservices/{{ID}}/servers/eligible endpoint need to be updated to utilize data from the new tables added by whatever PR resolves #4611.

This depends on #4611

Traffic Control components affected ...

• Documentation
• Traffic Ops
• Traffic Ops client (Go)

Acceptance Criteria

• /deliveryservices/{{ID}}/servers/eligible in API 2.x:
  • Uses the tables to provide the right legacy output for the GET method (should be unchanged)
• /deliveryservices/{{ID}}/servers/eligible in API 3.x:
  • Provides the proper representations of cache servers as outlined in the blueprint for the GET method
  • New behavior adheres to the API guidelines (where applicable)
  • All behavior changes have been documented
• The Traffic Ops Go client has been updated (v3 only) to return the new structures
  • Any associated unit and/or client/API integration tests have been updated to
    test the new structure and all pass.

See the Additional Server Interfaces blueprint for details.