Add Response Metadata Fields to HTTP Storage Plugin

In many http calls, the response code can contain meaningful information, however these fields are not available unless the call results in an error.

This PR adds four implicit metadata fields to the HTTP storage plugin which are:

• _response_code: The HTTP response code. This field is returned as an INT.
• _resposne_message: The response message.
• _response_protocol: The response protocol.
• _response_url: The actual URL that is sent to the API.

As these are implicit fields, they will not appear in star queries.