Added IO configuration, and changed default handler

> [EZP-23378](https://jira.ez.no/browse/EZP-23378)
> Status: in progress

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. Two handlers are available : `filesystem` and `legacy_kernel`.

## Other changes
- Removed unused in-memory handler

## TODO
- [ ] DI tests
- [ ] Doc (including migration, since those who use legacy need to update their configuration to use `legacy_kernel`
- [x] Figure out why `ezpublish.core.io.handler.legacy` isn't registered...
Comment added (the services at the end aren't overridden, btw).