✨ 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/greet_v2.go

@@ -0,0 +1,24 @@
+package greet
+
+import (
+	"context"
+	"github.com/rs/zerolog/log"
+)
+
+type ServiceV2 struct{}
+
+func NewServiceV2() *ServiceV2 {
+	return &ServiceV2{}
+}
+
+// GreetV2 returns a v2 greeting message for the given name.
+// If name is empty, it defaults to "friend".
+// This is the v2 implementation that returns "Hello my friend <name>!"
+func (s *ServiceV2) GreetV2(ctx context.Context, name string) string {
+	log.Trace().Ctx(ctx).Str("name", name).Msg("GreetV2 function called")
+
+	if name == "" {
+		return "Hello my friend!"
+	}
+	return "Hello my friend " + name + "!"
+}