Added IO configuration, and changed default handler

> [EZP-23378](https://jira.ez.no/browse/EZP-23378)
> Status: open for review

This mostly introduces semantic configuration for IO handlers, and makes filesystem the default handler.

## Semantic, siteaccess aware IO configuration
```
ez_io:
  system:
    default:
      filesystem
```

## New service tag
Adds a new service tag: `ezpublish.io_handler`, with an alias property for the name of the handler.

## TODO
- [ ] DI tests