DFS: 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))
> Status: open for review

Contains definition of existing (Core) services, as well as semantic configuration for the io handler + definition of the related services.

## Configuration example
```
ez_io:
  system:
    default:
      filesystem
```

Adds a new service tag: `ezpublish.io_handler`, with an alias property for the name of the handler.

## TODO
- [ ] DI tests
- [ ] Remove old files from CoreBundle
Actually, this might be wrong... I need to think further, I might have missed something here, it was late.