Add support for connector/task lifecycle ops to UI backend

These operations needs to be added:

• Connector pause, resume, and restart
• Task restart
• Get connector and task(s) status for connector (might exist already); this should show the actual status, exception in case of failures
• As a follow-up, some metrics, e.g. messages/sec, max lag in last min, five min etc. (or percentiles)

related Kafka Connect API KIP: https://cwiki.apache.org/confluence/display/KAFKA/KIP-52%3A+Connector+Control+APIs