Add an initial private capability for the JPA subsystem

At some point we want a public capability for JPA, so it can expose a formal contract to other subsystem (e.g. weld.) But sorting out what that capability exposes doesn't need to block adding a private capability first. Having the private capability will allow the JPA subsystem to properly expose the requirements it has on capabilities exposed by other subsystems. And that will allow Galleon to properly understand those relationships when it generates configurations as part of server configuration.

So this subtask is to add such a private capability, with the remaining work of sorting out what public contract it will provide leftover as part of the parent issue.

It's not a problem to take a private capability and in the future make it public. And it's not a problem to change what a private capability exposes, i.e. in preparation for making it public.