feat: allow specifying secrets for workflows

[lidavidm]

What's Changed

The new section looks like this:

[secrets]
SNOWFLAKE_DATABASE = { secret = "SNOWFLAKE_DATABASE", contexts = ["validate"] }
SNOWFLAKE_SCHEMA = { secret = "SNOWFLAKE_SCHEMA", contexts = ["validate"] }
SNOWFLAKE_URI = { secret = "SNOWFLAKE_URI", contexts = ["validate"] }

Also, use ubuntu-slim in a few places. This won't matter for public repos but should save a bit for private repos.

Closes #12.
Closes #35.

[lidavidm]

In action here: adbc-drivers/snowflake#53

[amoeba]

Nice, this looks great.