Convert expiration worker to a lambda

[CalvinRodo]

Description

Currently the expiration worker is run as part of the retrieval service.

While this is not having an impact on service performance it does introduce the possibility of a race condition.

This caused an issue where deleted OTKs were being counted multiple times. The issue has been resolved by using transactions but this caused a small increase in the number of DB connections needed.

Ideally this task should be handled by a microservice and run periodically during the day, it does not need to occur in the go microservices.

Expiration Server Code sits here: https://github.com/cds-snc/covid-alert-server/blob/master/pkg/workers/expiration.go

AC

• Write a lambda in chalice that will clean up the expired keys on a regular schedule
• lambda is monitered using x-ray
• lambda is deployed using TF

[CalvinRodo]

While this would be a nice to have this hasn't been an issue since this was opened, will close and re-open if required.