Create EncryptedRedisStructStorage

[zachmargolis]

A mixin for storing structs in Redis that are encrypted and have an expiration

Re: https://github.com/18F/identity-idp/pull/4213/files#r490421107

This PR is just an extraction/refactor of the existing code and interface we wrote for DocumentCaptureSessionResult

I personally think .store might make for a good instance method instead of a class method (see below) but I didn't want to "rock the boat" that much

struct = MyStruct.new(id: '1', a: 'a', b: 'b')
struct.store

# vs today:
MyStruct.store(id: '1', a: 'a', b: 'b')

[mitchellhenke]

looks great! 👍🏼

[zachmargolis]

Merging because it's just lint failures from the main branch that are failing

[esparta]

The following suggestions are only useful iif EncryptedRedisStructStorage.load is a hot path, as in used very frequently in the system. If not, please disregard it.

[esparta]

This method would create an array + string on every single call.

Suggested change

	[type.redis_key_prefix, id].join(':')
	type.redis_key_prefix + ":" id
	# or
	# "#{type.redis_key_prefix}:#{id}"

[esparta]

I would suggest to don't ask if the type respond to redis_key_prefix. IMO it would be programmatic error[1] by failing to follow the contract, and let it fall with something similar to this:

NoMethodError (undefined method `redis_key_prefix' for DocumentCaptureSessionResult:Class)

Is not as nice as the descriptive raise on line 53, but close enough to see where it's failing.

[1] Compared to user error.