feat: change social provider config to flat config

demo/nextjs/lib/auth.ts

@@ -75,14 +75,14 @@ export const auth = betterAuth({
 		}),
 		passkey(),
 	],
-	socialProvider: [
-		github({
+	socialProviders: {
+		github: {
 			clientId: process.env.GITHUB_CLIENT_ID || "",
 			clientSecret: process.env.GITHUB_CLIENT_SECRET || "",
-		}),
-		google({
+		},
+		google: {
 			clientId: process.env.GOOGLE_CLIENT_ID || "",
 			clientSecret: process.env.GOOGLE_CLIENT_SECRET || "",
-		}),
-	],
+		},
+	},
 });