Add widgets and alarms for Application Load Balancer resources

[eoinsha]

Create alarms for

• Application Load Balancer
• Target Groups (including Lambda targets)

There are a number of possible inputs to be considered.

1. All resources are in the CF stack (ALB, Listener, Listener Rule, Target Group) - in this case we can create all alarms/widgets ✅
2. Only ALB is in the stack - Just create the Load Balancer alarms/widget ✅
3. Only Target Group is in the stack - in this case, we can't get the Load Balancer Full Name (required for metrics dimensions, so we create nothing!) ⛔️
4. Target Group, Listener Rule and Listener are in the stack (no ALB) - this becomes challenging because we still need the LoadBalancerFullName for the metrics dimensions but it's not trivial to get for all possible LoadBalancerArns in the listener (SSM parameter, ImportValue, literals, etc.) - in this case, we won't create any alarms or widgets ⛔️

Diagram showing links between relevant resources in CloudFormation: