Refactored IO elements to EzPublishIOBundle

> [EZP-23328](https://jira.ez.no/browse/EZP-23328) (sub-task: [EZP-23380](https://jira.ez.no/browse/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.