Support multimodule projects

[davidsumegi]

CertUtils::toInputStream uses getResource instead of getResourceInputStream to open files like .cer or .p12.

This means that in a multimodule project, if multiple modules would like to use the files, it's not enough to put them in the parent's main/resources, but it requires extra copies to provide the same files to multiple modules.

[drallgood]

@davidsumegi Yep. Probably makes sense. Feel free to submit a PR.
In general the CertUtils is only a helper class you don't have to use.