v4 API: Expose Response interface

[n7studios]

Summary

Exposes the GuzzleHttp\Psr7\Response object via using getResponseInterface(), allowing the consumer to check e.g. a 2xx HTTP status code, header or other properties:

$result = $api->add_subscriber_to_form(12345, 'joe.bloggs@convertkit.com');
$code = $api->getResponseInterface()->getStatusCode(); // 200 OK if e.g. a subscriber already added to the specified form, 201 Created if the subscriber added to the specified form for the first time.

Testing

• testGetResponseInterface: Test that a valid PSR-7 response object is returned by getResponseInterface and can be interacted with.
• testClientInterfaceInjection: Test that a valid PSR-7 response object is returned by getResponseInterface when set_http_client is used (PR).

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[noelherrick]

Makes sense!