Keeping notifications API lambda warm (or reduce startup time)

Background context It has been noted that the response time from the notifications API can be quite lengthy. This is because there's hardly anyone using the API currently and so the execution environment goes cold. We should look at reducing the startup time and/or keeping the environment warm with an eventbridge scheduled event like we do with some of the other services

Specification

CD and PL to pair programme
Add Eventbridge event and rule to Notifications stack that keeps the Lambda warm every x minutes by calling the Swagger endpoint. This only needs to happen in the production environment.

reapit / foundations

Keeping notifications API lambda warm (or reduce startup time) #10602