Add ABI L2 support
I was surprised at how much this component didn't support L2. @rayg @wroberts we'll need to update G2G and maybe L1b and L2 processing to include a processing_level="l1b"
(or "l2") in the AMQP payload. The defaults will be fine for now. In hindsight I wish I never made processing level a thing that appeared anywhere...but here we are.
This MR adds tests and support for L2 products. It sets a default processing level environment variable to make the AMQP messages work even if the incoming message doesn't include it.