fix(terraform): gcp postgresql default values

[losisin]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Description

Fix checks when using default (undeclared) settings from GCP.

Fixes #3323

Edited policies

CKV_GCP_51, CKV_GCP_52, CKV_GCP_53 and CKV_GCP_54.

Description

When scanning terraform plan with checkov and running infrastructure with something like steampipe, result should match. Here is a list of all available PostgreSQL database flags with their respective default values: https://cloud.google.com/sql/docs/postgres/flags#list-flags-postgres

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[gruebel]

in general looks good, just need to fix the tests and then it is ready to be merged 🚀

[gruebel]

nice, great job 🥇