Rename alert field on Alert response to payload

[david-crespo]

While working on oxidecomputer/console#3320, we found the alert field on the Alert response a little awkward to talk about, a sign about naming that we try to take seriously when it's not too costly. Fortunately the alert list endpoint added in #11072 has not been in a release yet, so it's really no big deal to change it. Here we rename it to payload. @hawkw, @fakemonster, and I considered alternatives like data and details, but we're landing on payload.

data was a strong candidate because in the actual body we send to the webhook receiver, we call the whole thing the payload and we call the alert contents data there (see below). Still, I like that payload indicates it's the thing that is sent on somewhere. Somehow the metonymy there is much more tolerable than alert and Alert — maybe just because it's across two different objects rather than within the same one.

omicron/nexus/src/app/webhook.rs

Lines 497 to 504 in 9d95e0c

	#[derive(serde::Serialize, Debug)]
	struct Payload<'a> {
	alert_class: AlertClass,
	alert_version: u32,
	alert_id: AlertUuid,
	data: &'a serde_json::Value,
	delivery: DeliveryMetadata<'a>,
	}

[hawkw]

We discussed this a bit on Matrix, and I'm on board with the renaming. I had thought about calling it that when I added the API, but vacillated and went with alert instead; in hindsight I should have stuck with payload. Glad we got this in before shipping R23.