Add Subscriber API functions

[n7studios]

Summary

Adds the following API functions to provide API coverage for Subscribers

• update_subscriber()
• unsubscribe()

Outputs a PHP deprecated notice if using form_unsubscribe(), because:

• The function name is misleading; it doesn't just unsubscribe the email address from a form,
• The argument supports an array of options; unsubscribe() provides the specific supported argument (email address)

form_unsubscribe() is not removed at this time, to ensure we don't introduce breaking changes to the SDK. The deprecation notice serves as an early warning to developers to migrate to unsubscribe() as we will remove form_unsubscribe() at a later date.

Testing

• Tests for above functions included.

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
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• 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]

Looking over all these tests, I wonder if we should be throwing a custom exception and checking for that in case we replace GuzzleHttp. The error also would narrow things down if this gets used by other folks who might be using Guzzle for other HTTP requests.

[n7studios]

Looking over all these tests, I wonder if we should be throwing a custom exception and checking for that in case we replace GuzzleHttp. The error also would narrow things down if this gets used by other folks who might be using Guzzle for other HTTP requests.

Can be explored in a future PR, as:

1. Highly unlikely that we would replace Guzzle; it's one of the most popular PHP HTTP clients used
2. Throwing our own exceptions would provide more detail, although may be seen as a breaking change, which I'm not keen on introducing to a first stable version of this SDK. At present Guzzle's ClientException is thrown, and as a developer who uses the current ConvertKit PHP SDK, I'd continue to expect the same behaviour unless expressly told otherwise.
3. Developers are welcome to narrow down the cause of the error by inspecting the API's response e.g.:

try {
    $api->get_subscriber(12345);  
} catch (GuzzleHttp\Exception\ClientException $e) {
    $error = json_decode($e->getResponse()->getBody()->getContents());
    die($error->message); // Or however else they want to inspect the error message from the API and handle it.
}