Integrate sigstore/sigstore-maven-plugin into repo

[loosebazooka]

• Move sigstore-maven-plugin in sigstore/sigstore-java
• Make it a gradle module (and build with gradle :o! sorry)

TODO:

• Integrate publishing
• Update READMEs
• Deprecate all old maven sigstore plugin repos

[loosebazooka]

@vlsi there are some things with the gradle build logic I may not be completely following -- hoping you might have an idea if it looks okay.

[loosebazooka]

@vlsi I'm going to get this moving along, feel free to keep commenting if you see anything. I'll make changes post-merge.