Change StripeClient initialization

[ob-stripe]

r? @remi-stripe @richardm-stripe
cc @stripe/api-libraries

Change the StripeClient constructor to expose a simpler API.

The new constructor takes a single argument, which can be a string (the API key) or an array with various configuration values.

In most cases, users will use the string form:

$stripe = new \Stripe\StripeClient('sk_test_123');

If needed, it's easy to pass "advanced" configuration settings:

$stripe = new \Stripe\StripeClient([
    'api_key' => 'sk_test_123',
    'client_id' => 'ca_123',
]);

The configuration array approach is used by other PHP libraries, including Guzzle, the most popular PHP HTTP client library (https://github.com/guzzle/guzzle/blob/1cdd69a0cd361ee8b6ba5584e45db80c0ea8b31c/src/Client.php#L51). The main downside is that there is no standard PHPDoc syntax for defining the shape of the array, but I think PHPStan and other static analyzer tools are working on something like this, and it's not that bad to just use freeform text to expose the various configuration keys and their expected types.