Support for env vars and destinations setup

[alexanderkirtzel]

Instead of using hardcoded values, we should support env vars (e.g. datasetId or tableId)

[alexanderkirtzel]

For the firebase stack we want to add a folder and/or env files, where we can configure credentials and common settings.
Keep in mind, that the destinations will become configurable as well. Therefore a ts file might be preferable.
A destination might require an initial setup to be called once (e.g. for creating tables). Besides deploying the stack we want to also call the setup as well