✨ 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/config/config.go

@@ -17,6 +17,7 @@ type Config struct {
 	Shutdown  ShutdownConfig  `mapstructure:"shutdown"`
 	Logging   LoggingConfig   `mapstructure:"logging"`
 	Telemetry TelemetryConfig `mapstructure:"telemetry"`
+	API       APIConfig       `mapstructure:"api"`
 }
 
 // ServerConfig holds server-related configuration
@@ -46,6 +47,11 @@ type TelemetryConfig struct {
 	Sampler      SamplerConfig `mapstructure:"sampler"`
 }
 
+// APIConfig holds API version configuration
+type APIConfig struct {
+	V2Enabled bool `mapstructure:"v2_enabled"`
+}
+
 // SamplerConfig holds tracing sampler configuration
 type SamplerConfig struct {
 	Type  string  `mapstructure:"type"`
@@ -78,6 +84,9 @@ func LoadConfig() (*Config, error) {
 	v.SetDefault("telemetry.sampler.type", "parentbased_always_on")
 	v.SetDefault("telemetry.sampler.ratio", 1.0)
 
+	// API defaults
+	v.SetDefault("api.v2_enabled", false)
+
 	// Check for custom config file path via environment variable
 	if configFile := os.Getenv("DLC_CONFIG_FILE"); configFile != "" {
 		v.SetConfigFile(configFile)
@@ -118,6 +127,9 @@ func LoadConfig() (*Config, error) {
 	v.BindEnv("telemetry.sampler.type", "DLC_TELEMETRY_SAMPLER_TYPE")
 	v.BindEnv("telemetry.sampler.ratio", "DLC_TELEMETRY_SAMPLER_RATIO")
 
+	// API environment variables
+	v.BindEnv("api.v2_enabled", "DLC_API_V2_ENABLED")
+
 	// Unmarshal into Config struct
 	var config Config
 	if err := v.Unmarshal(&config); err != nil {
@@ -145,6 +157,7 @@ func LoadConfig() (*Config, error) {
 		Str("logging_output", config.Logging.Output).
 		Bool("telemetry_enabled", config.Telemetry.Enabled).
 		Str("telemetry_service", config.Telemetry.ServiceName).
+		Bool("api_v2_enabled", config.API.V2Enabled).
 		Msg("Configuration loaded")
 
 	return &config, nil
@@ -185,6 +198,11 @@ func (c *Config) GetSamplerRatio() float64 {
 	return c.Telemetry.Sampler.Ratio
 }
 
+// GetV2Enabled returns whether v2 API is enabled
+func (c *Config) GetV2Enabled() bool {
+	return c.API.V2Enabled
+}
+
 // GetLogLevel returns the logging level
 func (c *Config) GetLogLevel() string {
 	return c.Logging.Level