[RFE] Add common Java methods

Java has a lot of features that we now and then need to touch in our charms. Most commonly: managing certs in truststore, keys in keystore or JVM options.

My proposal is to add a new lib here that will be imported by charms needing java-related methods.

We can also use this single-point-of-touch to enforce certain security measures, such as which store encryptions are acceptable.

No file, test, or entry point is named. Start by clarifying which certificate, truststore, keystore, and JVM-option operations are required and which encryption policies are expected. Done means an agreed shared library API exists for the requested Java-related charm methods, with security requirements defined.

Written by the indexing model from the issue text.