Add a tool, rds-iam-psql, for developers to get a psql shell connected via RDS IAM auth

[andrew-corbalt]

PR Description

Adds a helper tool rds-iam-psql which uses AWS Creds and a few Postgres server related arguments to drop the developer into a psql session.

Tests:
Created psql session.
Verified search path args set the search path as expected.

PR Checklist

• New automated tests have been written to the extent possible.
• The code has been checked for structural/syntactic validity.
  • AMI/application: a build was performed
  • terraform changes: "terraform plan" checked on every affected environment
• (If applicable) the code has been manually tested on our infrastructure.
  • AMI/application: deployed an a test or dev environment
  • terraform changes: applied to test or dev environment
  • script: run against test or dev environment
• Likely failure points and new functionality have been identified and tested manually.
  Examples:
  • Application manually run in a way that triggers any new branches
  • AMI logged into and changes verified from login shell
• Pull request description includes a description of all the manual steps performed to accomplish the above.

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[hundt-corbalt]

cool!