Document minimum necessary permissions in testable form

[brabster]

See #66 (comment)

As a dbt-athena contributor
I want an IAM role defined in the source that describes the minimal permissions needed to execute dbt-athena
So that I can test against those permissions to detect and avoid breaking changes

As a dbt-athena consumer
I want an IAM role defined in the source that describes the minimal permissions needed to execute dbt-athena
So that I understand what permissions I need to make available to principals executing dbt-athena

Acceptance Criteria:

• IAM role exists describing minimal permissions
• Documentation references IAM role for contributors and consumers
• Documentation describes how to test with the role
• OPTIONAL make provides option to deploy the role to target AWS account
• OPTIONAL functional tests automatically assume the role