Cache signatures so that multiple runs of the mojo can reuse loaded signatures

[gnodet]

Loading JDK signatures can take more than half a second, so when the build contains lots of modules, it can save a lot of time

[gnodet]

This is interesting mainly in the context of mvnd where the mojo class loaders are reused. In a traditional maven build, the class loaders are deleted after each mojo run, so there would be no benefit.
@hboutemy @olamy