Improve "Private App" API usage

Overview/summary

Currently using this library when connecting to a private app is confusing.

It takes quite a long time to work out what you are doing, when it feels like it could be really simple.

Motivation

In our usecase, we only want to consume the Shopify admin api via REST, with a private app access token.

Currently to do that you have to have the following code:

Context::initialize(
  'xxx',                                          // Not used
  'yyy',                                          // Not used
  'read_orders, write_orders',                    // Not used
  'http://localhost',                             // Not used
  new FileSessionStorage('/tmp/php_sessions'),    // Not used
  'latest',
  false,                                          // Not used
  false,                                          // Needs to (incorrectly) be set to false
);

$rest = new Rest($domain, $storeApiToken);

This is not helped by the fact that there is a bug on this line:

https://github.com/Shopify/shopify-php-api/blob/main/src/Clients/Rest.php#L46

So you actually have to Context::initialize with "privateApp" to to false, so it uses the access token not the secret key.

       $headers[HttpHeaders::X_SHOPIFY_ACCESS_TOKEN] =
            Context::$IS_PRIVATE_APP ? Context::$API_SECRET_KEY : $this->accessToken;

There is barely any point in having to call Context::initialize, the only reason for doing it is to:

• Set Context::$IS_INITIALIZED (to prevent an exception)
• Default Context::$HTTP_CLIENT_FACTORY = new HttpClientFactory();

In an ideal situation I would just need to do this:

$rest = new Rest($domain, $storeApiToken);

Possible Improvements

• Remove the need to call Context::initialize
• Default the API version to 'unstable'
• Default the initialization of $HTTP_CLIENT_FACTORY if null
• Don't throw an exception during log if not initialized

Happy to contribute these changes, if you are welcome to receive them.

Start with src/Clients/Rest.php around the access-token header and trace Context::initialize and its initialization checks. Confirm the desired direct Rest($domain, $storeApiToken) flow, including the listed defaults and logging behavior, then verify the existing API behavior and tests still pass.