Support JSONAPI `application/vnd.api+json` media type

The **[JSON API specification](https://jsonapi.org)** uses `application/vnd.api+json` as its designated media type. This should work just like `application/json`, but Chalice will not parse the request's JSON payload into a Python object:

```python
@app.route("/someroute", methods=["POST"], content_types=["application/vnd.api+json"])
def someroute():
request = app.current_request
assert request.json_body is not None # Fails
```

Maybe we could keep a list for JSON related media types, just like `app.api.binary_types`?

Start by tracing how request.json_body handles content_types and compare that behavior with app.api.binary_types. Add coverage for application/vnd.api+json alongside application/json, and verify the example route receives a parsed Python object for that media type.

Written by the indexing model from the issue text.