feat(auth): implement OAuth 2.0 authorization code flow and dynamic client registration

- Add OAuth 2.0 support with authorization code flow and dynamic client registration.
- Introduce new handlers for OAuth metadata, client registration, authorization, and token issuance.
- Enhance authentication middleware to support OAuth client credentials.
- Create in-memory stores for authorization codes and tokens.
- Update configuration to include OAuth client details.
- Ensure validation checks for OAuth clients in the configuration.

internal/auth/dynamic_client_store.go

@@ -0,0 +1,62 @@
+package auth
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"sync"
+	"time"
+)
+
+// DynamicClient holds a dynamically registered OAuth client (RFC 7591).
+type DynamicClient struct {
+	ClientID     string
+	ClientName   string
+	RedirectURIs []string
+	CreatedAt    time.Time
+}
+
+// HasRedirectURI reports whether uri is registered for this client.
+func (c *DynamicClient) HasRedirectURI(uri string) bool {
+	for _, u := range c.RedirectURIs {
+		if u == uri {
+			return true
+		}
+	}
+	return false
+}
+
+// DynamicClientStore holds dynamically registered OAuth clients in memory.
+type DynamicClientStore struct {
+	mu      sync.RWMutex
+	clients map[string]DynamicClient
+}
+
+func NewDynamicClientStore() *DynamicClientStore {
+	return &DynamicClientStore{clients: make(map[string]DynamicClient)}
+}
+
+// Register creates a new client and returns it.
+func (s *DynamicClientStore) Register(name string, redirectURIs []string) (DynamicClient, error) {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		return DynamicClient{}, err
+	}
+	client := DynamicClient{
+		ClientID:     hex.EncodeToString(b),
+		ClientName:   name,
+		RedirectURIs: append([]string(nil), redirectURIs...),
+		CreatedAt:    time.Now(),
+	}
+	s.mu.Lock()
+	s.clients[client.ClientID] = client
+	s.mu.Unlock()
+	return client, nil
+}
+
+// Lookup returns the client for the given client_id.
+func (s *DynamicClientStore) Lookup(clientID string) (DynamicClient, bool) {
+	s.mu.RLock()
+	client, ok := s.clients[clientID]
+	s.mu.RUnlock()
+	return client, ok
+}