Add saas feature yml configs

What does this MR do and why?

• needed for extra verification and requirements for SaaS config file entries
• document and check. Based off of feature flag setup and docs.
• use stage_label from the gem.
• add ee features and saas features to the config files check in danger.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #426492 (closed)