Specify fields to receive in Application Webhook events

My team uses Application Webhooks to ingest data from Front. We generally only need to access certain fields in an event payload, with most of the data in the payload being discarded.

To help save on our ingress bandwidth costs (and Front's egress costs!) it would be helpful to allow us to specify what attributes we want to receive when configuring an Application Webhook.

As an example, we might want to ingest `inbound_received` events, but are only concerned with the message recipients and subject - not the body or attachments.
Omitting the body and attachments can in some cases save multiple MBs from being sent.