Provide a way to override query building logic

[Voileexperiments]

Currently __query_collection is double underscore prefixed, which means it takes some extra effort to override the logic for building the queries.

This makes the class completely inextensible and non-customizable. It's also very strange, since if I'm using postgres (compared to something infinitely easier to setup and use, like ChromaDB) to operate embedding data, obviously I'm trying to utilize its RDBMS features, which means I might want to customize my queries to join tables. With the current design it's not "intended" for users to do this in the first place, which makes the class very useless.