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:

• I added unittests
• I ran make check

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

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

[extrawurst]

since this basically undos the way ssh signing worked since 945ec45 i assume we can cleanup some of the now not used dependencies?

[b1nhack]

Okay, I'll check the code and try to make more optimizations.

[b1nhack]

No unused dependencies, but did some other code cleanup, fixed a test.