Package variable injection

[jakthom]

It would be very nice to be able to inject environment variables into packages, and therefore allow dynamic sourcing of things, or dynamic filtering, or whatever else one can imagine.

[movinfinex]

This is practically a must if you want to deploy somewhere with multiple environments (e.g., staging and production). Here are some ideas for features I'd find useful:

1. Adding a getenv() UDF like the one in the DuckDB CLI.
2. Generalising InjectLabelFromEnv() to work like os.ExpandEnv (but with escaping support), and using it for other config values.
3. Adding a vars config that defines SET VARIABLE variables for use in queries.

Option 1 would be a solution for queries that's safe from any injection/escaping issues. Option 2 would be unsafe in queries, but handy for things that aren't queries. Option 3 would just be handy for DRYing out code.

Are you open to PRs for any of the above?