refactor ingest CLI for better code reuse (#1846)
### Description
Much of the current CLI code is copy-paste across subcommands. To
alleviate this, most of the duplicate code was moved into base classes
for src and destination connector commands. This also allows for code
reuse when a destination command is called and it no longer has to jump
through hoops to dynamically recreate what _would_ have been called by a
source command.
The reason everything can't live in a single BaseCmd class is due to the
need for a dynamic map to the source command. This runs into a circular
dependency issue if it was all in one class. By splitting it into a
`BaseSrcCmd` and a `BaseDestCmd` class, this helps avoid that issue.
