feat(scheduler): ScheduleGroup (aws#26196)

This PR contains implementation of ScheduleGroup. 

A Schedule is the main resource in Amazon EventBridge Scheduler, this PR adds ScheduleGroup which can be used to group Schedules and on which Schedule depends.

Every AWS account comes with a default group for schedules. Customers can also create a custom groups to organise schedules that share a common purpose or belong to the same environment. 

Schedule has a property `group` that determines what group is the schedule associated with.

To be able to test adding schedules to the group I have added property `group` to private class `Schedule` and used `Lazy` functionality to be able to update `group` of the schedule dynamically. 

Implementation is based on RFC: https://github.com/aws/aws-cdk-rfcs/blob/master/text/0474-event-bridge-scheduler-l2.md

Advances aws#23394

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-scheduler-alpha/README.md

@@ -44,6 +44,7 @@ TODO: Schedule is not yet fully implemented. See section in [L2 Event Bridge Sch
 Only an L2 class is created that wraps the L1 class and handles the following properties:
 
 - schedule
+- schedule group
 - target (only LambdaInvoke is supported for now)
 - flexibleTimeWindow will be set to `{ mode: 'OFF' }`
 
@@ -97,7 +98,31 @@ const oneTimeSchedule = new Schedule(this, 'Schedule', {
 
 ### Grouping Schedules
 
-TODO: Group is not yet implemented. See section in [L2 Event Bridge Scheduler RFC](https://github.com/aws/aws-cdk-rfcs/blob/master/text/0474-event-bridge-scheduler-l2.md)
+Your AWS account comes with a default scheduler group. You can access default group in CDK with:
+
+```text
+const defaultGroup = Group.fromDefaultGroup(this, "DefaultGroup");
+```
+
+If not specified a schedule is added to the default group. However, you can also add the schedule to a custom scheduling group managed by you:
+
+```text
+const group = new Group(this, "Group", {
+    groupName: "MyGroup",
+});
+
+const target = new targets.LambdaInvoke(props.func, {
+    input: ScheduleTargetInput.fromObject({
+        "payload": "useful",
+    }),
+});
+
+new Schedule(this, 'Schedule', {
+    scheduleExpression: ScheduleExpression.rate(Duration.minutes(10)),
+    target,
+    group,
+});
+```
 
 ## Scheduler Targets
 
@@ -164,4 +189,28 @@ TODO: Not yet implemented. See section in [L2 Event Bridge Scheduler RFC](https:
 
 ### Metrics for a Group
 
-TODO: Not yet implemented. See section in [L2 Event Bridge Scheduler RFC](https://github.com/aws/aws-cdk-rfcs/blob/master/text/0474-event-bridge-scheduler-l2.md)
+To view metrics for a specific group you can use methods on class `Group`:
+
+```ts
+const group = new Group(this, "Group", {
+    groupName: "MyGroup",
+});
+
+new cloudwatch.Alarm(this, 'MyGroupErrorAlarm', {
+    metric: group.metricTargetErrors(),
+    evaluationPeriods: 1,
+    threshold: 0
+});
+
+// Or use default group
+const defaultGroup = Group.fromDefaultGroup(this, "DefaultGroup");
+new cloudwatch.Alarm(this, 'DefaultGroupErrorAlarm', {
+    metric: defaultGroup.metricTargetErrors(),
+    evaluationPeriods: 1,
+    threshold: 0
+});
+```
+
+See full list of metrics and their description at
+[Monitoring Using CloudWatch Metrics](https://docs.aws.amazon.com/scheduler/latest/UserGuide/monitoring-cloudwatch.html)
+in the *AWS Event Bridge Scheduler User Guide*.