Webhook (Lambda) on add/update/delete mutations

No, it is only post-mutation hooks.

Pre-hooks over lambda will add cost in terms of mutation latency, as they would require Dgraph to suspend the mutation until the network call from lambda doesn’t return.

I think, for validation, we should be supporting that through some directive that would allow listing down the constraints on the input in a manner that they can be executed on Dgraph Alpha itself.