Consider implementing caching of migration whitelist in SUMS proxy

[plittlewood-rpt]

The SUMS proxy is responsible for a huge volume of API transactions against the internal parameter API (circa 20 million a month). Now that we have already implemented logic in our lambdas to only initialize our DI containers once per Lambda container, we are in a position to also be able to locally cache the migration whitelist which is being constantly checked by this function. It would be fairly straightforwards to implement a timed cache locally inside the function. There is an obvious cost saving associated with doing this

Specification

• Fetch the migration whitelist each time a Lambda container is invoked (we only register DI once per container)
• Store the whitelist along with a timestamp
• Each time the function is invoked, check the cached version against the timestamp. If it's more than 5 minutes ago (perhaps make the timeout configurable), refetch it before continuing execution

[github-actions[bot]]

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