Support `estimatedBlocksUntilConfirmed` field

[sambarnes]

Thanks for jumping on the other typing issues i mentioned earlier! @aaronbarnardsound

Apologies for not seeing this one sooner, but a product need arose and I noticed that the estimatedBlocksUntilConfirmed field that's included in the webhook's base payload is also missing here.

I might have the bandwidth to submit a PR depending on your availability here