feat(sign): add support for external ssh signing

[b1nhack]

This Pull Request fixes/closes #2188, #2198.

It changes the following:

• Adds support for external bin signing with ssh keys

I followed the checklist:

• [x] I added unittests
• [x] I ran make check

thread 'tests::test_pre_commit_py' panicked at git2-hooks/src/lib.rs:471:9: assertion failed: res.is_ok()

• [x] I tested the overall application
• [ ] I added an appropriate item to the changelog