[refactor] Inference engines

[paxcema]

Changelog:

• [x] Reorganized codebase into inference engines
• [x] Current approach is now the "rule based" engine
• [x] Define new class abstractions
• [x] Move existing engine from method to class
• [x] Figure out if the current fan out approach is general enough for other engines.
• [x] Clean up API entry points
• [x] Refactor so that there are approach-specific reqs
• [x] Organize poetry deps into core + per-engine