Add support for directly-passing secrets to task library tasks

[jlowin]

Closes #1669 by adding support for passing values directly to various task library tasks.

This PR attempts to maintain all prior behavior, just raising a warning if the secret name is passed instead of a value.

In addition, it's a little noisy because it coincides with the release of a new version of black

[codecov]

Codecov Report

Merging #1673 into master will increase coverage by 0.03%.
The diff coverage is 97.67%.

[cicdw]

Only concern I have is that this introduces a large number of breaking changes; I know you did it so that you can detect when to raise the deprecation warning, but we should at least call this out in the Changelog.