[PM-1012] Feature access using context

[withinfocus]

Type of change

- [ ] Bug fix
- [X] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Methods for accessing feature flags as simple Booleans, integers, and strings, all using the current context.

Code changes

• src/Core/Constants.cs: Class for feature flag keys, as constants.
• src/Core/Services/IFeatureService.cs: Three new interface methods to retrieve values using context and defaults.
• src/Core/Services/Implementations/LaunchDarklyFeatureService.cs: Implementation using LaunchDarkly. Builds a context of the user and organization IDs when available (although the user context is truly needed) and respects default values for when offline.
• test/Core.Test/Services/LaunchDarklyFeatureServiceTests.cs: Tests to exercise the new methods with a generated user ID. Marked a few as skips so they can stick around as example implementations.

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• If making database changes - make sure you also update Entity Framework queries and/or migrations
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team