[FP-914] Ignore (unused) redrive_policy in SNS subscription lifecycle

[rv-jgentile]

Ref. [Assist] Sage's redrive policy is removed and re-added on TF apply

Welcome to my Ted Talk.

Why

• Currently topic subscriptions requiring a redrive policy are created alongside the app terraform, because redrive policy is not implemented in this module (or the module it was forked from - I checked).
• This module still creates the subscriptions, however, which causes it to remove the redrive policy from the resource that actually created it.

Confused? Me too, here's some terraform:

• Pay close attention: These are the same resource (note the id/arn), and they live in the state file in two different locations.

Exhibit A

$ terraform state show 'module.velocify.module.event-sns.aws_sns_topic_subscription.subscription[0]'
# module.velocify.module.event-sns.aws_sns_topic_subscription.subscription[0]:
resource "aws_sns_topic_subscription" "subscription" {
    arn                             = "arn:aws:sns:us-east-1:446051487243:qa-interest-event:9fefb62f-a378-42e4-a44d-b750675ac167"
    confirmation_timeout_in_minutes = 1
    confirmation_was_authenticated  = true
    endpoint                        = "arn:aws:lambda:us-east-1:446051487243:function:qa-velocify"
    endpoint_auto_confirms          = false
    id                              = "arn:aws:sns:us-east-1:446051487243:qa-interest-event:9fefb62f-a378-42e4-a44d-b750675ac167"
    owner_id                        = "446051487243"
    pending_confirmation            = false
    protocol                        = "lambda"
    raw_message_delivery            = false
    topic_arn                       = "arn:aws:sns:us-east-1:446051487243:qa-interest-event"
}

Exhibit B

$ terraform state show aws_sns_topic_subscription.invoke_with_sns
# aws_sns_topic_subscription.invoke_with_sns:
resource "aws_sns_topic_subscription" "invoke_with_sns" {
    arn                             = "arn:aws:sns:us-east-1:446051487243:qa-interest-event:9fefb62f-a378-42e4-a44d-b750675ac167"
    confirmation_timeout_in_minutes = 1
    confirmation_was_authenticated  = true
    endpoint                        = "arn:aws:lambda:us-east-1:446051487243:function:qa-velocify"
    endpoint_auto_confirms          = false
    id                              = "arn:aws:sns:us-east-1:446051487243:qa-interest-event:9fefb62f-a378-42e4-a44d-b750675ac167"
    owner_id                        = "446051487243"
    pending_confirmation            = false
    protocol                        = "lambda"
    raw_message_delivery            = false
    redrive_policy                  = jsonencode(
        {
            deadLetterTargetArn = "arn:aws:sqs:us-east-1:446051487243:qa-dead-letter"
        }
    )
    topic_arn                       = "arn:aws:sns:us-east-1:446051487243:qa-interest-event"
}

What does this PR do?

• This adds redrive_policy to a list of ignored changes in the sns submodule. The submodule will still function the same because redrive_policy has never actually been passed into it, so it's never actually been there, so it's always been implicitly ignored anyway.
• This allows application infra to declare ad-hoc subscriptions with a redrive policy and prevents this submodule from destroying that policy. Basically it doesn't know that it's there.