Add autoscaling policies and custom alarms to GuCDK

The Ophan team is halfway through the transition from CFN Yaml to GuCDK for one of their components, The Slab, and that's when we encountered lots of code to be written in Typescript for autoscaling rules (ScaleUpPolicy, ScaleDownPolicy) and 3 Cloudwatch alarms and each one of them references one of the autoscaling rules as an action. Also I want to mention that `Lag` is a custom metric. For more details check https://github.com/guardian/ophan/blob/c6321d02bc50723f0d46efb022e2305709fc826f/cloudformation/the-slab.cfn.yaml#L155-L220

It would have been easier if we are using `Step scaling` but unfortunately we are using `Simple scaling` which required lots of AWS CDK code.

I paired with @akash1810 to find the best way to accomplish that and this is how we did it https://github.com/guardian/ophan/pull/5326/commits/72d00fba5b2ba2f3dfa5ef72420a65683c1a8955 but I asked if such scenarios could be done in GuCDK so that is why I wanted to add visibility to that requirement and if it is something that DevX team would consider.

Compare the CloudFormation section linked at cloudformation/the-slab.cfn.yaml#L155-L220 with the implementation in the linked Ophan pull request. Then inspect GuCDK's existing component entry points to determine whether simple-scaling policies and custom CloudWatch alarms fit its abstractions; done means a decided, documented approach or supported capability.

Written by the indexing model from the issue text.