[CT-1578] Unify secret management

[nathaniel-may]

Once secrets are in memory, secrets are sometimes indistinguishable to developers from benign strings. Secrets should be reliably distinguishable from benign strings for the lifetime of a dbt run.

The ambiguity makes it necessary to remember to
scrub secrets naively before making any output.

Secrets are not consumed the same way when provided in files and environment variables which drives some of this ambiguity:

• If the password is in profiles.yml, we will attempt to jinja render the password which makes certain valid passwords throw an exception because they include partial jinja syntax.
• environment variables have a secret prefix and are replaced with a secret placeholder

Passwords should be treated the same regardless of their origin. Developers should be aware when they are working with secrets enough to not rely on naive output scrubbing.