passReqToCallback support missing from Digest strategy

The **Basic** stragegy implements the **passReqToCallback** option, but support for this is missing in the **Digest** strategy.
Perhaps both callbacks, **secret** and **validate**, should honour this option.

Compare the Digest strategy's callback handling with the Basic strategy's passReqToCallback support. Check how the secret and validate callbacks are invoked, then verify that both honor the option without breaking existing Digest authentication behavior.

Written by the indexing model from the issue text.