✨ 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

features/greet.feature

@@ -10,4 +10,14 @@ Feature: Greet Service
   Scenario: Personalized greeting
     Given the server is running
     When I request a greeting for "John"
-    Then the response should be "{\"message\":\"Hello John!\"}"
+    Then the response should be "{\"message\":\"Hello John!\"}"
+
+  Scenario: v2 greeting with JSON POST request
+    Given the server is running with v2 enabled
+    When I send a POST request to v2 greet with name "John"
+    Then the response should be "{\"message\":\"Hello my friend John!\"}"
+
+  Scenario: v2 default greeting with empty name
+    Given the server is running with v2 enabled
+    When I send a POST request to v2 greet with name ""
+    Then the response should be "{\"message\":\"Hello my friend!\"}"