fix(terraform): Amazon RDS uses an AWS KMS key by default

[nikpivkin]

Source: https://github.com/aquasecurity/tfsec/issues/2086

Amazon RDS uses the AWS managed key for new DB instance. Therefore, we need to improve the rule, and if Performance Insights is enabled, but the KMS key is not specified, then warn the user that he is not using the client key, and not say that performance insights is enabled without protection.

We can create 2 rules instead of 1. One to verify that Performance Insights is enabled and the second to verify the use of the client key. For example, how it is implemented to verify the EC2 volume encryption:

• https://avd.aquasec.com/misconfig/aws/ec2/avd-aws-0027/
• https://avd.aquasec.com/misconfig/aws/ec2/avd-aws-0026/