custom ca-cert import into java keystore

Enable the import of one or more custom CA certificate(s) into java keystore e.g. for metadata download via https or api calls to systems without public trusted certificates.

A workaround already exists in the base image: ff7c72f6

todo:

• Implement solution in entrypoint -> iterate over /usr/src/shibboleth-idp-certs and import all found certs into java keystore goal:
• Certs can be copied in customer overlay Dockerfile or directly via mount to /usr/src/shibboleth-idp-certs