[Alerting] move always-available action variables to alerting/common

[pmuellr]

In PR #59756 action variables were added as a new property of alert types. There are a handful of "always available" action variables such as alertId and alertName. The definitions of these are duplicated in the alerting and triggers_actions_ui plugins:

kibana/x-pack/plugins/alerting/server/task_runner/transform_action_params.ts

Lines 38 to 47 in ce1836b

	const variables = {
	alertId,
	alertName,
	spaceId,
	tags,
	alertInstanceId,
	context,
	state,
	};
	return Mustache.render(value, variables);

kibana/x-pack/plugins/triggers_actions_ui/public/application/lib/action_variables.ts

Lines 27 to 42 in ce1836b

	function getAlwaysProvidedActionVariables(): ActionVariable[] {
	const result: ActionVariable[] = [];
	result.push({
	name: 'alertId',
	description: i18n.translate('xpack.triggersActionsUI.actionVariables.alertIdLabel', {
	defaultMessage: 'The id of the alert.',
	}),
	});
	result.push({
	name: 'alertName',
	description: i18n.translate('xpack.triggersActionsUI.actionVariables.alertNameLabel', {
	defaultMessage: 'The name of the alert.',
	}),
	});

To prevent these from diverging in the future, we should move them to the alerting/common folder, and then reference them in the files above from there.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)