Fix webhook registration rejecting events unknown to the Event enum

[smety]

Problem

Item::setEvent() (RegisterNewWebhookRequest) hard-validates event strings against the bundled Webhook\Event enum and throws InvalidArgumentException for anything it does not know. The enum currently contains 56 events, while the webhook event types code list publishes 113 — the SDK therefore refuses to register events the API itself
accepts.

Reproduction

Sdk::registerNewWebhook(['data' => [[
    'event' => 'eshop:settingsInformation', // published in the code list
    'url' => 'https://example.com/webhook',
]]]);
// InvalidArgumentException: "eshop:settingsInformation" is not valid webhook event type value

The same applies to the whole product:, category:, productVariant:* families (including all mass events), order:paid, supplier:* and ~50 more.

1. Item::setEvent() no longer validates strings against the enum. The API is the validation authority — an unknown event is rejected server-side with a proper error response. Client-side validation against
   a bundled list inevitably drifts and blocks legitimate registrations. Event enum values keep working exactly as before.
2. Webhook\Event synced with the current code list (+55 cases). Existing cases are untouched — no BC break. Events marked beta in the code list (product:, productVariant:, productVariantPrice:, incl. mass variants) are included, consistent with the beta stock: events that have been part of the enum already.
3. Unit tests for Item::setEvent() — enum input, known string, unknown/future string, serialization.

[xmalda]

Thank you for suggesting this change.
We fully agree with it.

We have processed the change in the original repository and it will be merged as part of the PR #103
I am closing this pull request as duplicity.