Specific syntax for teklia workers in update_system_workers

Refs https://redmine.teklia.com/issues/7497

As we are the main provider of system workers (and overall Arkindex workers) for the foreseeable future, we need a more customized solution to configure default system workers in Arkindex.

The update_system_workers command need to support creating user configurations, as they are defined in source repositories. Instead of duplicating that information and having to maintain it twice, we'll simply reference these workers as teklia_worker in arkindex/system_workers.yml:

features:
    <ARKINDEX SLUG>:
        teklia_worker:
            name: arkindex/workers/xxxx (known as <NAME>)
            version: <VERSION>
            slug: <WORKER SLUG>

We'll still support the current configuration of image + command, but the teklia_worker will allow us:

1. to reference workers set on https://gitlab.teklia.com exclusively
2. whose docker image is hosted on registry.gitlab.teklia.com
3. that have a .arkindex.yml file

The workflow will then be:

1. retrieve (without credentials) the .arkindex.yml file at https://gitlab.teklia.com/<NAME>/-/raw/<VERSION>/.arkindex.yml
2. parse it to retrieve the <WORKER SLUG>
3. use registry.gitlab.teklia.com/<NAME>:<VERSION> as docker image
4. use defined user configuration & command