Add AI explainer feature

[anth-volk]

As outlined in the planning document created for this purpose

[MaxGhenis]

Per our discussion, we'll start with a model in which the user can request that we store the result set, in which case we return a result_id (could change the name) which is itself the intersection of household and API version (we would separate household_id from result_id either here or in a future update). This is optional and defaults to False, representing the current behavior that does not store info on our servers.

They then pass that result_id along with the variable name, and we return the AI summary. This would default to non-streaming, though users could pass stream=True to receive a stream.