Enable polling for content for Image Source

[anik120]

Introduce new field spec.source.image.pollInterval to allow users to specify how often catalogD should check for new content in the remote host. For more details including salient implementation points, please read the RFC: https://docs.google.com/document/d/1iWSrWL9pYRJ5Ua3VYErkK1Q2lAusBUeDCh66Ew4lDbQ/edit?usp=sharing