Implement tasks sub-commands

[Richard Case]

Implement sub-commands to work with tasks directly. We probably need 2 new sub-commands:

1. connectctl tasks get which given a cluster and connector name will return the tasks for that connector. The results could be output as JSON or as a table. If a task id is supplied then a specific task for that cluster will be output if not all tasks for that cluster.
   See

• https://docs.confluent.io/current/connect/references/restapi.html#get--connectors-(string-name)-tasks
• https://docs.confluent.io/current/connect/references/restapi.html#get--connectors-(string-name)-tasks-(int-taskid)-status

2. connectctl tasks restart which given a cluster, connector name and task id will restart a individual task.
   See: https://docs.confluent.io/current/connect/references/restapi.html#post--connectors-(string-name)-tasks-(int-taskid)-restart