Informix Connector not available

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).

Start with the `lb4 datasource` command to see how available connectors are presented, then review the Connectors reference and related files. Confirm the Informix connector is listed by the command and update the documentation so the connector is covered; the linked connector issue may provide implementation context.

Written by the indexing model from the issue text.