SNS topics should be encrypted too

## Background

Currently SNS topics are not encrypted and thus are flagged as a security issue.

## Desired Change

An equivalent KMS key generated for SNS. Here is an example pulled from `kms.tf` to encrypt SQS.

```hcl
// KMS key for server-side encryption (SSE) of SQS
resource "aws_kms_key" "sse_sqs" {
description = "BinaryAlert Server-Side Encryption - SQS"
enable_key_rotation = true

tags = {
Name = var.tagged_name
}

policy = data.aws_iam_policy_document.kms_allow_s3.json
}

resource "aws_kms_alias" "sse_sqs_alias" {
name = "alias/${var.name_prefix}_binaryalert_sse_sqs"
target_key_id = aws_kms_key.sse_sqs.key_id
}
```