IVGCVSW-3596 Register the dynamic backends in the BackendRegistry

IVGCVSW-3596 Register the dynamic backends in the BackendRegistry

 * Added getter for the factory function in the DynamicBackend class
 * Added new RegisterDynamicBackends method in utils class
 * Added dynamic backend registration process in the Runtime class
 * Added new dummy dynamic backend objects for testing
 * Added unit tests for dynamic backend registration
 * Added convenience methods to BackendId

Change-Id: I01e147d1d6f01bf56747ad946f73f867af5770c4
Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>