Add external id support for organizations (#1232)

## Description

Deserializes it from the api response, allows creating and updating
organizations with an external id, and provides a
getOrganizationByExternalId function.

## Documentation

Does this require changes to the WorkOS Docs? E.g. the [API
Reference](https://workos.com/docs/reference) or code snippets need
updates.

```
[x] Yes
```

Documentation coming soon