github action: create action for validating omni command directories

[XaF]

This should:

• Allow to check if binary files all have metadata, and allow to parameterize which metadata are mandatory (default: category, help)
• Allow to check if the command's --help parameter calls omni's help for that command (otherwise warning)
• Optionally warn if non-executable files are in the directory tree

This could be evolved in the future to include any other checks that we feel pertinent to run in a CI when wanting to provide omni commands for a repository.
This partially depends on #27 if we want to check for metadata for all the files.