Add Alembic

[Owen-CH-Leung]

Add Alembic as DB migration tool

What is this Python project?

Alembic provides for the creation, management, and invocation of change management scripts for a relational database, using SQLAlchemy as the underlying engine. It's widely used by a variety of projects such as airflow

What's the difference between this Python project and similar ones?

Not aware of any other libraries that can provide the same utility as this package does

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[Owen-CH-Leung]

@vinta any chance of getting this PR merged ? Thanks