Add EnsureSchema function to create database schema

[andrew-corbalt]

PR Description

As we refactor for RDS IAM, ConnectWithSchema() will be deprecated and this EnsureSchema() helper function will handle the creation of a schema if it does not exist which was previously handled within ConnectWithSchema()

Tests:
This function was used to perform migrations against a local postgres DB.

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]

LGTM! Might be worth adding a comment that this requires CREATE privileges even if the schema already exists and it would be a no-op.