Support customer managed KMS keys for dynamo DB

## Background

Currently the dynamoDB is encrypted using AWS managed keys. The `aws_dynamodb_table` resource permits use of encrypting this resource with a customer managed key as documented below.

https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/dynamodb_table#server_side_encryption

## Desired Change

Add new Terraform variable `dynamodb_kms_key_arn` (default value `null`) to allow the user to pass in a CMK key to `aws_dynamodb_table.binaryalert_yara_matches`.

Start by locating the Terraform definition of aws_dynamodb_table.binaryalert_yara_matches and the variable declarations for this deployment. Trace how existing variables reach the resource, then verify that the new optional KMS ARN is accepted and applied through a Terraform plan without changing the default behavior.

Written by the indexing model from the issue text.