Provide abstraction for Environments

[l-monninger]

Summary

The need for Environments is conceptually motivated in the README.md and pragmatically motivated in code under #151.

Note

These Environments would allow the loading of necessary resources for running Migrations and Migrations in turn should roughly provide the resources for Checks under the notion of Contexts

To build the Environments API, we recommend thinking down from a Config API which would allow specifications of the following form:

mtma-migrate-node migrate select --environment -- --environment.something-needed value --environment.something-else-needed other_value