Setup local PGP signing of published artifacts

[cortinico]

Setting up local signing of our artifacts.

To test that everything works fine @arturbosch you should set two properties: signingKey with the
ascii armored version of the GPG private key, and signingPwd with the passphrase.
Ideally once you publishToMavenLocal you should have also signatures for every artifact.

Fixes #2883

[codecov]

Codecov Report

Merging #2893 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2893   +/-   ##
=========================================
  Coverage     80.18%   80.18%           
  Complexity     2425     2425           
=========================================
  Files           421      421           
  Lines          7369     7369           
  Branches       1349     1349           
=========================================
  Hits           5909     5909           
  Misses          761      761           
  Partials        699      699           

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