[alerts] add explicit versions to alert payloads

[hawkw]

This branch adds an explicit notion of version numbers for alert payloads. This is implemented by changing the entry points for publishing alerts (either via Nexus::alert_publish or SitrepBuilder::request_alert for FM) to take a new AlertPayload trait implemented by types which describe alert payloads. These must provide an alert class and version constant, in addition to implementing serde::Serialize and JsonSchema. Eventually we will also use these to generate the schemas for each version of the alert so that they can be communicated to potential consumers. For now, though, we are at least enforcing that a given schema always provides the same alert class and version number (rather than being able to pass in any arbitrary class).

Now, when an alert is created, the schema version is recorded in the alert record. We backfill any existing alert records as being version 0. This doesn't actually matter because, you know, there aren't any, but we may as well do the right thing there. When an alert is delivered to a webhook receiver, the version number is included in a JSON field and a new x-oxide-alert-version header.