Duckplyr should allow users to set s3 secret tokens etc.

[Tmonster]

adjacent to #37 duckplyr does not expose any functionality to allow users to set their s3 secret keys etc. as described here. Basically users cannot execute sql against the default duckdb connection through duckplyr.

https://duckdb.org/docs/guides/import/s3_import.html

Might be nice to have functions like set_s3_keys(access_key, secret_access_key) and set_s3_session_token(session_token).

We can brainstorm sometime

[krlmlr]

Would duckdb pick it up from environment variables if unset?