Add variable skip_s3_notification to manage outside the module.

[theherk]

Since bucket notifications can only be managed once, if an implementer wants additional notifications on the bucket, they must be managed outside this module. If you give this variable as true, you must add a bucket notification to the lambda given in outputs as scan_lambda_function_arn. See this issue (#510) on the provider for more details on the topic.

This patch does not include updated README.md because I see that terraform_docs is not included in the pre-commit configuration, so I assume it is done by your pipeline.

[theherk]

I went ahead and updated the docs using:

terraform-docs markdown table --indent 2 --output-mode inject --output-file README.md --output-template '<!-- BEGIN_TF_DOCS -->
  {{ .Content }}
  <!-- END_TF_DOCS -->' .

since the build wanted to push back to my fork rather than update after merge in your source.