feat: implement webhook setup call parsing and response handling

[ag84ark]

Implementing the webhook using Webhook::parse(...) would fail on the initial setup on the website, as it sends a test call to see if it's working.
In this PR we take care of that call as well, as now it would return a 422.

@sandervanhooft Also need to publish after merge.

[ag84ark]

I think this also influences vatly-fluent-php if your not using the functionality from here directly

[sandervanhooft]

Superseded by the follow-up work. PR #30 targeted the temporary setup-call payload shape, but the backend now emits a standard event, so the replacement removes the special-case parser path and handles setup deliveries as ordinary objects.

[sandervanhooft]

Superseded by the follow-up feat/webhook-setup-event-v1 work. PR #30 targeted the temporary setup-call payload shape, but the backend now emits a standard webhook.setup event, so the replacement removes the special-case parser path and handles setup deliveries as ordinary WebhookPayload objects.