JMXUtil: add utility to create "jmx.objectname" map for service registration

there are a few places across the oak code that would benefit from a utility method in JMXUtil that allows to build a map with one key jmx.objectname and a hashtable as value.

see e.g. org.apache.jackrabbit.oak.spi.security.authentication.external.impl.ExternalLoginModuleFactory.