Adding a JSON API interface to fetch data in the JSON format. It abides with the JSON command and its implementations by other modules like mod_verto.
It outputs the exactly same column names as the data in CLI.
Various listing of mod_callcenter data such as agents, queues, members, tiers are as follows below.

- Listing of agents
Command: `json {"command": "callcenter_config", "format": "pretty", "data": {"arguments":"agent list"}}`

- Listing of queues
Command: `json {"command": "callcenter_config", "format": "pretty", "data": {"arguments":"queue list"}}`

- Listing of agents for a queue
Command: `json {"command": "callcenter_config", "data": {"arguments":"queue list agents", "queue_name":"support@default"}}`
(`queue_name` is a required)

- Listing of members(callers) for a queue
Command: `json {"command": "callcenter_config", "data": {"arguments":"queue list members", "queue_name":"support@default"}}`

- Listing of tiers for a queue
Command: `json {"command": "callcenter_config","data": {"arguments":"queue list tiers", "queue_name":"support@default"}}`

- Listing of members(callers)
Command: `json {"command": "callcenter_config","data": {"arguments":"member list"}}`

- Listing of tiers
Command: `json {"command": "callcenter_config","data": {"arguments":"tier list"}}`

It handles various errors and also returns back response in JSON as follow
E.g. Send a command without a required field (`queue_name`)
Command: `json {"command": "callcenter_config", "data": {"arguments":"queue list agents"}}`
Output: `{"command":"callcenter_config","data":{"arguments":"queue list agents"},"status":"error","message":"The command returned an error","response":{"error":"Missing data attribute: queue_name"}}`