Handle sandbox notifications

This endpoint returns a HTTP status code of 200 for authorized requests. This is used for testing purposes. Note that the authorization methods for each of the v1/api/notifications/<PSP> endpoints uses different authorization methods and a successful response from this endpoint does not guarantee that the other notifications endpoints are working as expected. It does provide assurance that the requests are being correctly proxied to Connector and that Connector is responding. <br>Requests are authorised either via the source IP address extracted from the HTTP x-forwarded-for header against the expected CIDRs from SANDBOX_ALLOWED_CIDRS or by validating the secret provided via the HTTP Authorization header against the secret within SANDBOX_AUTH_TOKEN. The latter use of the HTTP Authorization header provides a means to test the endpoint without needing to send the request from a fixed IP address.<br>The request body is not deserialised or processed in any way.