Using custom environment variables, on metabase SQL Query

### Feature requests and proposals

I am currently having a problem with a SQL Question that has an sql statement for decrypting a column. As of now (devs would only be the users for this) we are adding the keys every time we run the SQL Query Question.

Request: Using custom environment variable inside SQL Query Questions would be an additional security removing the key from plain sight, but still allowing users to query the encrypted column that has the data

Would also like to know if there is another way around this problem, currently looking for other solutions.

No files, tests, or entry points are identified. Start by reviewing how SQL Query Questions receive parameters and how Metabase handles custom environment variables, then confirm the security model with maintainers. Done means a defined, supported way to use the secret without exposing it in the query or interface, with tests covering the approved behavior.

Written by the indexing model from the issue text.