Support plain leveldb source type for KATC

[RebeccaMahany]

Some extension data is stored in a plain leveldb (not indexeddb on top of leveldb) database. We would like to be able to query this via the KATC tables.

Add new leveldb source type to https://github.com/kolide/launcher/blob/main/ee/katc/config.go, and support reading a key-value leveldb. We can use some of the basic code from https://github.com/kolide/launcher/tree/main/ee/indexeddb -- there's no encoding and no nesting present, so we can simply iterate over the database keys instead of having to find the correct database and object store.