Refactored IO elements to EzPublishIOBundle

See EZP-23328 (sub-task: EZP-23380)

Contains definition of existing (Core) services, as well as semantic configuration + parsing for the io handler.

ez_io:
  system:
    default:
      filesystem

Adds a new service tag: ezpublish.io_handler, with an alias property for the name of the handler.