Add integration tests that use a private maven repository

Proposal

The Java runtime does not utilize the ca certificates stored within the /etc/ssl/certs/ca-certificates.crt. Instead, it uses the keystore within $JAVA_HOME/lib/security/cacerts, or $JAVA_HOME/jre/lib/security/cacerts if using Java 8. This presents an issue where the analyzer (written in Go) might be able to communicate with the server using the custom ssl certificate, but Maven or Gradle might not. Testing this will require adding a package to our maven and gradle test project from a private maven registry that uses a self-signed certificate. This setup will ensure that gemnasium-maven, sbomgen-gradle, and sbomgen-maven are all able to correctly verify the cert presented by the private registry and connect to it.

Additional details

The custom-ca-ssl-server might be able to proxy requests for maven packages to maven central. If the custom ca test restricts network access to the maven central repository, then this will ensure that maven/gradle use https://ssl-test/ to resolve the packages.

/cc @adamcohen @willmeek