Informix Connector not available

Personne n'a encore pris cette issue.

Referring to https://github.com/strongloop/loopback-connector-informix/issues/56

The Informix Connector is not part of the tooling.
It should be available by default so informix users can take the advantage of using loopback with their systems

## Acceptance Criteria
- [ ] when running `lb4 datasource`, the informix connectors show up as one of the available connectors.
- [ ] update [Connectors reference](https://loopback.io/doc/en/lb4/Connectors-reference.html) and any related files

Related to: strongloop/loopback-next#3714

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## 🎆 Hacktoberfest 2020

Greetings :wave: to all Hacktoberfest 2020 participants!

Here are few tips 👀 to make your start easier, see also #6456:

- Before you start working on this issue, please leave a comment to let others know.
- This issue consists of several tasks to work on, it may feel like a too big effort to undertake. Don't worry! It's perfectly fine to pick just one item from the list and leave the rest for somebody else. In fact, we prefer to have a dedicated pull request for each part, to make it easier for us to review the changes and get the pull request landed faster. Baby steps FTW! Remember, every little helps.
- If you are new to GitHub pull requests, then you can learn about the process in [Submitting a pull request to LoopBack 4](https://loopback.io/doc/en/lb4/submitting_a_pr.html).
- If this is your first contribution to LoopBack, then please take a look at our [Developer guide](https://loopback.io/doc/en/lb4/code-contrib-lb4.html)
- Feel free to ask for help in `#loopback-contributors` channel, you can join our Slack workspace [here](https://join.slack.com/t/loopbackio/shared_invite/zt-8lbow73r-SKAKz61Vdao~_rGf91pcsw).

Commencez par la commande `lb4 datasource` pour voir comment les connecteurs disponibles sont présentés, puis examinez la référence Connectors et les fichiers associés. Vérifiez que le connecteur Informix est listé par la commande et mettez à jour la documentation afin que le connecteur soit couvert ; l’issue du connecteur liée peut fournir un contexte d’implémentation.

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