Switch out authorizer DynamoDb cache for Redis cache

[plittlewood-rpt]

Background context The DynamoDb bill has gone up quite considerably since the addition of the gateway authorizer cache due to the number of read/write requests. It would be far cheaper to run this cache using Elasticache instead, which costs about $50 a month. We already use a redis cache to manage request concurrency which copes with the number of requests

Specification

• Project: Reapit.Lambdas.GatewayAuthorizer
• Introduce new implementation of IGatewayCache that uses Elasticache/Redis instead. Same caching logic should apply
• Add new Redis cache to CF stack as we should use a separate one to the one used for Request concurrency so that it doesn't get overloaded. The DynamoDb table can be dropped post-release, but NOT at the same time (two separate updates to the stack)
• Remember that the permissions for the function will need to be updated
• Remember this should be released in all regions (two in production)
• In the event we cannot communicate with the cache, we should run the full auth routine to prevent any downtime

[github-actions[bot]]

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