✨ feat: implement API v2 with feature flag control

- Added /api/v2/greet POST endpoint with JSON request/response

- Implemented ServiceV2 with Hello my friend <name>! greeting format

- Added api.v2_enabled feature flag (default: false)

- Extended BDD tests to cover v2 scenarios

- Maintained full backward compatibility with v1 API

- Added DLC_API_V2_ENABLED environment variable support

- Created ADR 0010-api-v2-feature-flag.md

- Updated configuration system to support API versioning

adr/README.md

@@ -68,6 +68,7 @@ Chosen option: "[Option 1]" because [justification]
 * [0007-opentelemetry-integration.md](0007-opentelemetry-integration.md) - Integrate OpenTelemetry for distributed tracing
 * [0008-bdd-testing.md](0008-bdd-testing.md) - Adopt BDD with Godog for behavioral testing
 * [0009-hybrid-testing-approach.md](0009-hybrid-testing-approach.md) - Combine BDD and Swagger-based testing
+* [0010-api-v2-feature-flag.md](0010-api-v2-feature-flag.md) - API v2 implementation with feature flag control
 
 ## How to Add a New ADR