Separate goals for signing with GPG and BC

### New feature, improvement proposal

Lots of goal parameters of `sign` either apply to Bouncycastle or GPG signing. In order to simplify documentation and parametrization those should be separate goals (with a common base class).

Start by locating the existing sign goal and reviewing which parameters apply to Bouncy Castle versus GPG signing. Define separate goals with a shared base class, then verify that the documentation and parameterization clearly reflect the separation.

Written by the indexing model from the issue text.