Build the notification a matched subscription receives (REQUIRED — both paths).
OptionalextractThe external-resource key(s) an incoming payload concerns (webhook providers).
OptionalnameOptionalpollPoll seam (poll providers). Omit ⇒ webhook-only (no host-DO alarm).
OptionalpollOptional poll cadence; a pure webhook provider omits it.
OptionalverifySignature check (webhook providers). Omit ⇒ the provider accepts no webhooks.
Config for the generic webhook provider — the core
WebhookSignalProviderknobs (extractResourceId,buildNotification) PLUS the signature check core omits (a generic webhook has no signing scheme; a real one — GitHub — does).