refactor(api): restructured API for type safety, maintainability and docs generation

- These changes makes the API incombatible with previous versions
- Added new types for error handling, success responses, and health checks.
- Updated health check logic to utilize the new types for better clarity and structure.
- Refactored existing handlers to improve response consistency and error handling.
- Updated Makefile to include a new target for generating API types from Swagger.
- Updated "new agent" API to respond an encrypted cert pair

internal/api/v1/agent/common.go

@@ -0,0 +1,67 @@
+package agentapi
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"sync/atomic"
+	"time"
+
+	"github.com/rs/zerolog/log"
+	"github.com/yusing/go-proxy/agent/pkg/agent"
+)
+
+type PEMPairResponse struct {
+	Cert string `json:"cert" format:"base64"`
+	Key  string `json:"key" format:"base64"`
+} // @name PEMPairResponse
+
+var encryptionKey atomic.Value
+
+const rotateKeyInterval = 15 * time.Minute
+
+func init() {
+	if err := rotateKey(); err != nil {
+		log.Panic().Err(err).Msg("failed to generate encryption key")
+	}
+	go func() {
+		for range time.Tick(rotateKeyInterval) {
+			if err := rotateKey(); err != nil {
+				log.Error().Err(err).Msg("failed to rotate encryption key")
+			}
+		}
+	}()
+}
+
+func getEncryptionKey() []byte {
+	return encryptionKey.Load().([]byte)
+}
+
+func rotateKey() error {
+	// generate a random 32 bytes key
+	key := make([]byte, 32)
+	if _, err := rand.Read(key); err != nil {
+		return err
+	}
+	encryptionKey.Store(key)
+	return nil
+}
+
+func toPEMPairResponse(encPEMPair agent.PEMPair) PEMPairResponse {
+	return PEMPairResponse{
+		Cert: base64.StdEncoding.EncodeToString(encPEMPair.Cert),
+		Key:  base64.StdEncoding.EncodeToString(encPEMPair.Key),
+	}
+}
+
+func fromEncryptedPEMPairResponse(pemPair PEMPairResponse) (agent.PEMPair, error) {
+	encCert, err := base64.StdEncoding.DecodeString(pemPair.Cert)
+	if err != nil {
+		return agent.PEMPair{}, err
+	}
+	encKey, err := base64.StdEncoding.DecodeString(pemPair.Key)
+	if err != nil {
+		return agent.PEMPair{}, err
+	}
+	pair := agent.PEMPair{Cert: encCert, Key: encKey}
+	return pair.Decrypt(getEncryptionKey())
+}