Give entities custom IDs (collections, documents, functions, etc)

This was discussed in the Discord server, and I'm bringing the discussion here for better tracking.

Proposal


It would be helpful if we could create collections and documents with custom IDs. Many document based databases (MongoDB for example) allow for this and it makes for really great DX.

Challenges and discussion points


Some initial challenges and things to discuss brought up by @eldadfux were:

1. How the API is going to look?
2. Which entities (collections, documents, files, users, functions, etc...) will be entitled to custom ID and how can we create a consistent API for all of them