Move common abstractions from Balta into this repository

[lsulak]

See https://github.com/AbsaOSS/balta and look for 'mag' submodule.

The thing is, that we have:

• fa-db, for representing and easier work with DB functions in relational world, proven to work with Postgres, from Scala
• balta, for testing such functions against database but writing tests (as well as preconditions / postconditions for test cases) in Scala
• ultet, a database migration tool, written in Scala, but embed-able in any CI/CD in the future
• mag - some if not all of these three will use common DB abstractions - definition of table, column, SQL expression, etc. This library mag is serving as a common stable layer, meaning that mag has very little dependencies in itself, but other of our libraries are depending on it.