fallback model/providers (#259)

Co-authored-by: zuolingxuan <zuolingxuan@bytedance.com>

src/lib/providers.ts

@@ -29,6 +29,8 @@ export interface ProviderConfig {
   type: ProviderType;
   baseUrl?: string;
   model?: string;
+  fallbackModels?: string[];
+  fallbackProviderIds?: string[];
   enabled: boolean;
   createdAt: string;
   updatedAt: string;