Define and implement EventSource SPI

Current process of registration of EventSource requires several steps
for application to become event source.

These steps are: exporting node to event source topology and providing (registering) an RPC implementation of EventSourceService RPCs to MD-SAL.

These steps are currently manually implemented in NetconfEventSource / NetconfEventSourceManaget

In order to ease development of EventSources, convenience SPI and implementation
should be provided, which will expose method such
as

interface EventSourceRegistry

{ EventSourceRegistration registerEventSource(EventSourceService impl); }

which will return EventSourceRegistration (subclass of abstract object registration) with following:

InstanceIdentifier - path assigned to event source in event source topology.
close() - when invoked event source will be removed and registration of event source topology.

Proposed SPI contract is not definite, and should be iterated upon, to provide
best APIs. Available.

Instance of EventSourceRegistry should be available via Config Subsystem and
name of EventSource topology should be configurable.