✨ 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

pkg/greet/api_v2.go

@@ -0,0 +1,68 @@
+package greet
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/rs/zerolog/log"
+)
+
+type GreeterV2 interface {
+	GreetV2(ctx context.Context, name string) string
+}
+
+type ApiV2Greet interface {
+	RegisterRoutes(router chi.Router)
+}
+
+type apiV2GreetHandler struct {
+	greeter GreeterV2
+}
+
+func NewApiV2GreetHandler(greeter GreeterV2) ApiV2Greet {
+	return &apiV2GreetHandler{greeter: greeter}
+}
+
+func (h *apiV2GreetHandler) RegisterRoutes(router chi.Router) {
+	log.Trace().Msg("Registering v2 greet routes")
+	router.Post("/", h.handleGreetPost)
+	log.Trace().Msg("v2 Greet routes registered")
+}
+
+type greetRequest struct {
+	Name string `json:"name"`
+}
+
+type greetResponse struct {
+	Message string `json:"message"`
+}
+
+func (h *apiV2GreetHandler) handleGreetPost(w http.ResponseWriter, r *http.Request) {
+	// Read request body
+	body, err := io.ReadAll(r.Body)
+	if err != nil {
+		http.Error(w, `{"error":"failed to read request body"}`, http.StatusBadRequest)
+		return
+	}
+
+	// Parse JSON
+	var req greetRequest
+	if err := json.Unmarshal(body, &req); err != nil {
+		http.Error(w, `{"error":"invalid JSON format"}`, http.StatusBadRequest)
+		return
+	}
+
+	// Call service
+	message := h.greeter.GreetV2(r.Context(), req.Name)
+
+	// Write response
+	h.writeJSONResponse(w, message)
+}
+
+func (h *apiV2GreetHandler) writeJSONResponse(w http.ResponseWriter, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(greetResponse{Message: message})
+}