Consolidate mechanism for reading environment variables

We currently have about 4 ways to read environment variables:
1. click
2. invocation context
* provides a level of isolation between dbt command invocations (e.g. in dbt Runner)
* isolated test environments
* (to confirm specifics) only reads `DBT_SECRET` and `DBT_`-prefixed environment variables.
3. via `os` builtin package

Let's explore a standard recommended way for doing this, even if we may not be able to get around using click for parsing CLI options. Most likely we should just move away from (3) and to (2), and keep (1).

A note... we could also explore using pydantic for this use case, but unclear what the benefits would be over (2).