Add SSM-based feature flags

[lorenyu]

Ticket

Resolves #781

Changes

• Add modules/feature_flags
• Add feature_flags_config in app-config/env-config/feature_flags.tf with ability to override per environment via feature_flags_override
• Create feature flags as SSM parameters in infra/<app_name>/service/feature_flags.tf
• Add feature flags as environment variables with FF_ prefix

Context for reviewers

Re-adding feature flag support using SSM params and environment variables

Testing

see navapbc/platform-test#178

[doshitan]

Not blocking, but given this is tied to SSM/returns ARNs, I wonder if we should name the module like ssm_feature_flags or feature_flags__ssm (assuming we eventually develop a "feature_flag" interface of which this is one implementation) or something?

[lorenyu]

Hmm. It'd be nice to have module names be independent of cloud service provider, so we can have AWS and Azure modules be the same. Interesting idea, I'll leave it as is for now and we can think about ti more.