Business Flags - create common module

Currently we have a Flags class wrapper around launch darkly feature flags for the backend code that is duplicated across several services. This ticket is to create a common one that covers all the needed functionality and replace the local copy in each service.

Notes:
- we want to decouple it from the User model, without losing user/account targeting
- should be able to pass in user and account context
- move RequestContext class out of legal-api and business-registry-dissolution and decide whether or not to put it in common or flags package (should also be decoupled from User model)

Start by locating the duplicated Flags implementations across the backend services, then inspect RequestContext in legal-api and business-registry-dissolution. Define the common module boundary and verify that the local copies can be replaced while preserving user and account targeting without coupling to User.

Written by the indexing model from the issue text.