[webhook output] Make response timeout configurable

When testing webhook delivery to the Filebeat http_endpoint and requesting a delivery acknowledgement, it can take many seconds for the request to complete. elastic/stream has a hard-coded HTTP timeout of 1s which isn't long enough. So make the timeout configurable.

https://github.com/elastic/stream/blob/2969e284784011cd50e156af7acfb844bd7cc35a/internal/output/webhook/webhook.go#L35