fix: gnokey sign should sign transactions

[zivkovicmilos]

Description

This PR fixes the signing logic of gnokey sign, to now actually sign a transaction if the user's signature is missing.
Additionally, it cleans up the code, and adds coverage tests.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.24%. Comparing base (2f72f84) to head (4260cea).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1976      +/-   ##
==========================================
+ Coverage   45.63%   48.24%   +2.60%     
==========================================
  Files         480      408      -72     
  Lines       68817    62260    -6557     
==========================================
- Hits        31408    30039    -1369     
+ Misses      34795    29717    -5078     
+ Partials     2614     2504     -110     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.