OFREP endpoints

Need to add to our core API the endpoints as specified in the [OFREP](https://github.com/open-feature/protocol) so Flagsmith becomes compliant:

- `/ofrep/v1/evaluate/flags/{key}` - evaluate a single feature flag
- `/ofrep/v1/evaluate/flags` - bulk evaluate all feature flags

Full specification in the [OFREP OpenAPI document](https://github.com/open-feature/protocol/blob/main/service/openapi.yaml).

> [!NOTE]
> This epic is pending Server Side Events support in the bulk endpoint. OFREP states it's optional, so it will be brought to discussion for integrating with our Real Time Updates (for enterprise).

Start by reading the OFREP OpenAPI specification linked in the issue and tracing the core API entry points for single and bulk feature-flag evaluation. Done means both `/ofrep/v1/evaluate/flags/{key}` and `/ofrep/v1/evaluate/flags` comply with the specified responses; Server-Side Events support is explicitly pending discussion.

Written by the indexing model from the issue text.