PackageSigning - "nuget.exe sign" - Allow package authors to sign packages

[mishra14]

Goal: As part of the larger package signing task, we need to add support to allow package authors to sign their packages before publishing.

Spec: In review - #5907

[mishra14]

Current implementation for fake/mock signing is into the feature branch. Will get back to this after the signing APIs are ready. Updating estimate to reflect the new milestone.