IAM RDS cross account role support

[andrew-corbalt]

PR Description

This change allows the RDS IAM library to optionally specify a cross account role to assume before generating creds.
This allows RDS IAM auth to an RDS instance that lives in a different AWS account from where this library is being used.

Tests:
Ran professor-mac server locally and verified it was possible to get valid creds on DSO Metrics using a professor-mac role

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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