Reduce threshold for supporting 'readable' destinations

[aaronsteers]

Currently, the threshold for supporting a destination as "readable" is a very high bar. It requires that the destination is fully writeable as a cache, and also that we have a valid/tested mapping of cache config to destination config.

A path to reducing this threshold might look something like the following:

1. Eliminate the requirement for a SQL processor definition - instead relying on the get_sqlalchemy_url() implementation, which lives at the Cache class layer.
2. Bring the mapping of cache-config to destination-config, and vice versa, into the Cache class.