Add ability to specify dependencies without foreign keys

[mlb6]

With this PR I'm trying to fulfill the need that our projects require to be able use SnakeDumper. We have a more than 300 tables with billions rows. Most of the tables have foreign keys, but some dont have them. We cannot only rely on foreign keys to look for dependencies between tables. Because of this, some tables are dumped completely. We cannot dump everything as it would be too slow.

The aim of this PR is to provide :

• the ability to specify dependencies without foreign keys.
• the ability to specify "dynamic" dependencies. (Look at the demo.yaml or the unit test for example)
• the ability to use AND and OR operators as filters
• fix the isNull and isNotNull filter

This pull request is still a draft (requires more testing, and reviews from my team), but comments are welcome.