Implement `connectors config` sub-commands

[richardcase]

We need to implement commands to get and update the config of a connector.

I think we need 2 new commands:

1. connectctl connectors config get which is related to https://docs.confluent.io/current/connect/references/restapi.html#get--connectors-(string-name)-config
   2, connectctl connectors config update which is related to https://docs.confluent.io/current/connect/references/restapi.html#put--connectors-(string-name)-config