[Infrastructure UI][Rules] Specify recovery context for Metric Threshold rule type

[simianhacker]

The alerting framework has recently merged a PR to provide rule executors with the ability to specify context for recovered alerts. Details are available in the PR and in the alerting README but essentially,

1. Rule types should set doesSetRecoveryContext to true on the rule type. This allows context variables to appear in the action variable list in the UI for the recovered action group.
2. Inside the rule executor, when the executor has finished scheduling actions for active alerts, the following code should be added:

// Call done() to gain access to recovery utils
// If `doesSetRecoveryContext` is set to `false`, getRecoveredAlerts() returns an empty list
const { getRecoveredAlerts } = alertsFactory.done();

for (const alert of getRecoveredAlerts()) {
	const alertId = alert.getId();
	alert.setContext({
		CONTEXT HERE!
	})
}

Note that currently, we expect the recovered context to match the existing typing for context variables on the rule.

Related to: #126279

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)