[Feature Request] Support SearchKey as an Environment Variable

Having the `searchKey` in the clear in `engine.json` is not the best approaches from a security point of view.

It would be great if the `searchKey` can be read at runtime from an environment variable.

As an example the `AppSearchAPIConnector` connector would be something like the following:

```
const connector = new AppSearchAPIConnector({
searchKey: process.env.REACT_APP_APPSEARCH_SEARCHKEY,
engineName,
hostIdentifier,
endpointBase
});
```

Would like to get your opinion on this before performing any work. Ready to submit a PR for this feature 😄