Add support for complex file_id

[enkidulan]

First of all, many thanks for creating and maintaining this library. It's my first opportunity to contribute to it, so I want to let you know that I appreciate your work.

The changes in this PR are somewhat similar to the logic introduced in https://github.com/jowilf/sqlalchemy-file/pull/149 and https://github.com/jowilf/sqlalchemy-file/pull/154 that adds support for complex store names.

Complex store names are a great feature, but they don't cover cases where content on storage is grouped by any of the arbitrary properties (like user_id, post_id, or both). This PR adds support for complex file_id, which allows for a more flexible way of organizing files.

Let me know what you think, I'm happy to add more test coverage or update the PR as you see fit.