Include confluent-hub in the Docker image

[MichaelDrogalis]

Is your feature request related to a problem? Please describe.

Today, it's cumbersome to install new connectors to run in embedded mode. You need to either preinstall confluent-hub or download the connector jar yourself.

Describe the solution you'd like

We should embed confluent-hub into the Docker image so that connectors can be downloaded with ease, with something like the following:

docker run -v /mnt/hub/plugins:plugins confluentinc/ksqldb-server confluent-hub install < connector >

This is nice because you need not pre-pull the image. It will download if this is its first invocation.