[Spike] Allow extensions to contribute custom convention for environment-specific operational configuration

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**Timeboxed to 2 weeks**; don't spend extra time if done earlier

Different platforms use different ways for configuring operational aspects of application in test/dev/production. LB4 should make it easy to write extensions to support these different platforms.

The goal of this spike is to find out and document what's possible today, identify gaps, propose solutions and create a list of follow-up tasks.

Few examples of operational config:
- configure datasources from environment variables
- use a faster but less-secure hashing algorithm in dev/test (a hashing algorithm is used e.g. to store user passwords)
- apply datasource configuration provided by `VCAP_SERVICES` env variable in CloudFoundry/IBM Cloud - see https://github.com/strongloop/loopback-next/pull/1574

Possibly related: https://github.com/strongloop/loopback-next/pull/983 and https://github.com/strongloop/loopback-next/issues/1396

Commencez par examiner la prise en charge actuelle des extensions LoopBack 4 et de la configuration opérationnelle, puis lisez les pull requests associés 1574 et 983 ainsi que l’issue 1396. Documentez ce qui est possible aujourd’hui, identifiez les lacunes, proposez des solutions et répertoriez les tâches de suivi concrètes ; l’achèvement correspond au résultat documenté du spike dans le délai de deux semaines.

Rédigé par le modèle d'indexation à partir du texte de l'issue.