OGM-389 Moving configuration API from OgmSessionFactory to OgmConfiguration and OgmConfigurer

* Established OgmConfigurer callback for passing a complete configuration during bootstrapping, thus making the SF immutable
* Added OgmConfiguration#configureOptions(DatastoreConfiguration) for ad-hoc configurations without the need for implementing an OgmConfigurer
* Added DatastoreConfiguration#getConfigurationBuilder() and DatastoreProvider#getDatastoreConfigurationType() which represent the link between a datastore and its context types
* Removed ConfigurationBuilderService as the configuration will not be changed after SF bootstrap
OGM-389 Extracting shared test model classes from into separate package