Support command-based SecretGenerator DataSource

[jonasrmichel]

The secretGenerator.commands field was replaced by a new DataSource abstraction in v2.0.3 (#703).

The commands field provided secret hydration via shell commands, which was useful for fetching secrets from an arbitrary source (e.g., a hosted secret store).

The new DataSource mechanism only seems to support sourcing secrets stored in files. This approach is arguably simpler, but requires secrets be present on the file system, which is limiting from an administrative perspective.

I propose a shell-based DataSource implementation be provided to support the old style of command-sourced secrets.

[jonasrmichel]

Closing in favor of #760