Added Rego-based Filter Cloud Function for false positives

[onetwopunch]

• New Filter Cloud Function that integrates with OPA Rego
• Tests for the above
• New function in services for marking findings as INACTIVE
• Updated README with architecture diagram, instructions for filter usage, and Terraform inputs
• Added Terraform input for SCC Notifications and module to programmatically add this with a local-exec since it's required for the filtering functionality.
• Added sample Rego policies and corresponding tests
• Updated all Cloud Functions to use Go1.13 (since it was required for Rego)
• Added required GCP_PROJECT variable to each cloud function since that was removed after go111
• New top-level config directory for both automation YAML and filters (BREAKING CHANGE)
• Updated root level of source directory in router since it changed in go113
• Refactored Router Execute into smaller functions since it was failing go-cyclo as part of pre-commit
• Added a Makefile to make testing easier and added in gosec for security findings
• Validated all changes in real environment by triggering alerts and ensuring those alerts were either filtered or passed along to the router and executed.

[tomscript]

this is really fantastic thanks again Ryan!

[tomscript]

Ryan did you see the build failed? i wonder if it uses the old ver of golang?

https://github.com/GoogleCloudPlatform/security-response-automation/actions/runs/368990058

[onetwopunch]

Ah yes it looks like the Github action is using Go 1.11. I didn't even realize that was checked since it wasn't set up to block a merge. Lemme make a separate PR for this

[onetwopunch]

For future readers, the issue Tom pointed out about tests failing has been fixed in #200 and was due to Go version and plugin version being out of date in the github workflow.