Implement read and write support for zero-payload artifacts

Acceptance criteria:

• Mender-artifact must be able to write an artifact with zero payloads
• It must also be able to read this artifact
• It must be tested with the client to make sure it can also read and install it correctly
  • This can be a manual test for now. It will be tested automatically later on with the test in MEN-2585 and MEN-5594.
• The artifact_depends and artifact_provides headers in the artifact formation specification must be changed to accept free form string pairs, just like the version in type-info.
  • Remember to update the spec as well, not just the code!
  • There must be command line options to fill these headers, just like there is a --depends and --provides option when writing a payload (like in the write module-image help page). If you are unsure about this one, best to discuss the flags with the client team.

The motivation for having zero payloads is to install a bootstrap artifact with only artifact_provides and artifact_name headers.