Allow binaryalert to scan buckets created outside of module

[jdheyburn]

Background

Users may want binaryalert to scan buckets that were created outside of the module. It should support this functionality.

Weirdly, it looks like this functionality was partly included as documented here, via the variables:

• external_s3_bucket_resources
• external_kms_key_resources

But a more automated way should be implemented so the user doesn't need to change anything.

Desired Change

Create new variable external_s3_bucket_arns that will notify the analyzer SQS queue on new objects created. Adjust permissions accordingly.