new signing property outside of targets

[gabrielittner]

The property on signing doesn't really make sense. For maven-publish it was never used and for the old plugin the implementation was kind of hacky by checking which the upload task was going to be executed. This replaces the old property with a single one that is used for all targets. It defaults to a gradle property so you could still dynamically change it when running certain upload tasks.

I will update the readme after both this and #100 were merged

[codecov]

Codecov Report

Merging #101 into master will decrease coverage by 0.35%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master    #101      +/-   ##
===========================================
- Coverage     36.76%   36.4%   -0.36%     
  Complexity       49      49              
===========================================
  Files            21      21              
  Lines           408     412       +4     
  Branches         68      68              
===========================================
  Hits            150     150              
- Misses          230     234       +4     
  Partials         28      28

Impacted Files	Coverage Δ	Complexity Δ
...vanniktech/maven/publish/BaseMavenPublishPlugin.kt	62.16% <0%> (ø)	4 <0> (ø)	⬇️
...vanniktech/maven/publish/MavenPublishConfigurer.kt	0% <0%> (ø)	0 <0> (ø)	⬇️

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[vanniktech]

Yup makes more sense