[UIMA-6444] Automatically sign Eclipse plugins during release builds

[reckart]

What's in the PR

• Attempting to embed PGP signatures into the Eclipse artifacts
• Disabled due to a Tycho bug embedding the FULL public keychain of the release manager
• Also, didn't get Eclipse to recognize the embedded signatures yet (did work with a minimal example I built separately though...)
• WIP
• See discussion in the Tycho forums

How to test manually

• ...

Automatic testing

• [ ] PR adds/updates unit tests

Documentation

• [ ] PR adds/updates documentation

Organizational

• [ ] PR adds/updates dependencies. _{^{Only dependencies under approved licenses are allowed. LICENSE and NOTICE files in the respective modules where dependencies have been added as well as in the project root have been updated.}}