feat(dart): Add Google Sign-In template

dart/sign_in_with_google/.gitignore

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+.dart_tool/
+.packages
+pubspec.lock

dart/sign_in_with_google/README.md

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+# sign-in-with-google
+
+This function:
+
+1. Verifies a Google ID token obtained from the client application.
+1. If a user with matching id or email doesn't exist, a new user will be created.
+1. The user's email will be verified if Google has verified it and it hasn't been already in Appwrite.
+1. A token will be returned allowing the user to exchange the token for a session via `account.createSession()`.
+
+> Note: This function verifies the Google ID token by validating its cryptographic signature using Google's public keys (JWK format), as recommended in the [official documentation](https://developers.google.com/identity/gsi/web/guides/verify-google-id-token). The function also validates the token's audience, issuer, and expiry claims.
+
+## 🧰 Usage
+
+### POST /
+
+**Headers**
+
+The Content-Type header must be set to `application/json` so that the request body can be properly parsed as JSON.
+
+* `Content-Type`: `application/json`
+
+**Request**
+
+This function accepts:
+
+* `idToken` (required) - Google ID token obtained from the client-side Google Sign-In flow
+
+Sample request body:
+
+```json
+{
+    "idToken": "eyJhbGciOiJSUzI1NiIsImtpZCI6IjdlMzA3..."
+}
+```
+
+**Response**
+
+This function returns:
+
+* `secret` - `secret` to be passed to `account.createSession()` to create a session
+* `userId` - `userId` to be passed to `account.createSession()` to create a session
+* `expire` - ISO formatted timestamp for when the secret expires
+
+Sample `200` Response:
+
+```json
+{
+  "secret": "0cbdd4fd7638e0f3f55871adf2256f8f42f6faa01c9300e482c9a585b76611343dee8562ce4421b1cf9e9de6f8341fb2286499cb7992d02accd2dc699211008c",
+  "userId": "112345678901234567890",
+  "expire": "2025-07-15T00:10:21.345+00:00"
+}
+```
+
+## ⚙️ Configuration
+
+| Setting           | Value                       |
+| ----------------- | ---------------             |
+| Runtime           | Dart (3.5)                 |
+| Entrypoint        | `lib/main.dart`             |
+| Build Commands    | `dart pub get`              |
+| Permissions       | `any`                       |
+| Timeout (Seconds) | 15                          |
+| Scopes            | `users.read`, `users.write` |
+
+## 🔒 Environment Variables
+
+The following environment variables are required:
+
+* `GOOGLE_CLIENT_ID` - Your Google OAuth 2.0 Client ID from the Google Cloud Console
+
+## 📱 Client-Side Integration
+
+To obtain the Google ID token from your client application, use the [google_sign_in](https://pub.dev/packages/google_sign_in) package:
+
+```dart
+import 'package:google_sign_in/google_sign_in.dart';
+
+final GoogleSignIn _googleSignIn = GoogleSignIn(
+  clientId: 'YOUR_CLIENT_ID.apps.googleusercontent.com',
+);
+
+Future<void> signInWithGoogle() async {
+  try {
+    final GoogleSignInAccount? account = await _googleSignIn.signIn();
+    if (account == null) return;
+
+    final GoogleSignInAuthentication auth = await account.authentication;
+    final String? idToken = auth.idToken;
+
+    if (idToken != null) {
+      // Send the idToken to your Appwrite function
+      final response = await http.post(
+        Uri.parse('YOUR_FUNCTION_URL'),
+        headers: {'Content-Type': 'application/json'},
+        body: jsonEncode({'idToken': idToken}),
+      );
+      // Handle the response
+    }
+  } catch (error) {
+    print('Error signing in with Google: $error');
+  }
+}
+```
+
+## 🔐 Security Notes
+
+* The Google ID token's cryptographic signature is verified using Google's public keys (RSA)
+* The correct public key is selected using the `kid` (key ID) from the JWT header
+* The token's audience (client ID) is verified to match your application
+* The token's issuer is verified to be Google (`accounts.google.com`)
+* The token's expiration time is checked to ensure it hasn't expired
+* Email verification status from Google is honored in Appwrite

dart/sign_in_with_google/analysis_options.yaml

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+include: package:lints/recommended.yaml