Allow providers other than PolicyModelsProvider

Allow providers other than PolicyModelsProvider

This change separates the database initiation and close functions of the
DatabasePolicyModelsProviderImpl class into an abstract class that does
the database DAO initiation and closing, and a domain specific part,
that does the manipulation of the policy models in the database.

This means that other providers, that wish to manipulate other objects
in the database, can specialize the AbstractModelsProvider class and use
its common DAO initiation and clsoing functionality, but can have their
own domain specific code.

The abstract class also now supports the Closeable interface.

Note that the exiting JUnit tests provide 100% coverage on
AbstractModelsProvider.

Issue-ID: POLICY-2971
Change-Id: Ibe59c02ab127ddbd9355d892b94a5d315507a8e6
Signed-off-by: liamfallon <liam.fallon@est.tech>