Add an RDS offer

[bclodius]

A few notes about this PR

• database_edition is only valid for Oracle and SQL Server, without this parameter I was facing hash collisions
• license_model has different values based on the database_engine and sometimes multiple options for a database_engine; so made it required.

[bclodius]

@garrettheel

1. I'll certainly add some test cases for reserved_hourly and reserved_upfront

2. I'll review the errors for mypy for 3.6 and fix

3. The signing of CLA worked on retry

[bclodius]

Thanks 👍 I plan to add more in the coming months.