Embed SQLite Asset and SQLite Asset - CSV

[gilzoide]

It would be simpler if the functionality of SQLite Asset package resided in this main package, making it a one-stop shop for using SQLite in Unity. Using SQLite databases as assets is quite useful and it seems people are trying to do it manually instead of relying on ready-made functionality that this optional package brings.

We might as well bring along SQLite Asset - CSV for extra goodies.

Note on naming conventions: SQLite-net uses "SQLite", while SQLite Asset uses "Sqlite". We should use the same naming convention as this package already uses when copying code, so we should change any namespaces and class names to use "SQLite".