Add feature indirection to SOS tool

[Laren-AWS]

Currently, SOS is set up to publish a chapter for each service client, but some services (RDS is an example) subdivide their service into multiple "feature" guides, even though the API is all in the same service client. For these services, we need to be able to publish a chapter for each feature guide.

For example, RDS has two main guides: RDS and Aurora. We need:

• aurora_code_examples_chapter: the Aurora scenario and actions.
• rds_code_examples_chapter: the RDS instance scenario and actions.

The solution is likely to add a little bit of metadata for this kind of service and update to the tool to write separate chapters for this case (needs some research on the exact mechanism).