Make Signers reusable

### Affected version

all

### Bug description

The fact that maven-gpg-plugin project is "one shot" (one module) and it produces `maven-plugin` packaging, makes it fully non reusable, while we do have code in here that we (currently duplicate) want to reuse, like Resolver 2.x BC signing in maven-resolver-generator-gpg module.

This issue is just a discussion starter, to discuss why is having 1 module projects BAD (as for reusability, but same for testing reasons -- you must rebuild and rerun all the tests on whole matrix, forcing build itself to be kept on same output java level - java 8 in this case as build product carries. Effectively this prevents plugin from building with Maven 4 as it cannot run on Java 8, while testing should happen on Java 8 as well)

Start by comparing the duplicated Resolver 2.x BC signing code in maven-resolver-generator-gpg with the one-module maven-gpg-plugin project and its current testing setup. The issue does not name files or tests; completion would require an agreed design that enables signer reuse while preserving Java 8 testing and allowing the build to run with Maven 4.

Written by the indexing model from the issue text.