[Stack Monitoring] Add KQL validation to Stack Monitoring alerts

[simianhacker]

In #111663 we added KQL filters to the stack monitoring alerts. We need to add a condition that will prevent the user from saving any alerts with invalid KQL filters. We also suppressed the JSON parsing of the filterQuery in the rules like:

kibana/x-pack/plugins/monitoring/server/lib/alerts/fetch_cluster_health.ts

Lines 63 to 70 in 75fcfe1

	try {
	if (filterQuery) {
	const filterQueryObject = JSON.parse(filterQuery);
	params.body.query.bool.filter.push(filterQueryObject);
	}
	} catch (e) {
	// meh
	}

After giving it some thought we should just let those errors bubble up.

Related issues and PRs:

• [Metrics UI] Metrics threshold stores malformed KQL as an empty filter #119416
• [Monitoring] Add KQL filter bar to alerts #111663

[elasticmachine]

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