refactor(firestore): standalone googleapis_firestore package and multi-db support for firestore (#121)

* docs: update documentation link for sending messages to topic conditions

* refactor: update constructors to use internal factory methods for AppCheck, Auth, Firestore, Messaging, and SecurityRules

* update README

* wip refactor google_cloud_firestore into its own package

* wip refactor google_cloud_firestore files to streamline package structure

* refactor: rename files and update imports for googleapis_firestore package

* docs: add googleapis_firestore package documentation section to index.html

* feat: add multi-database support to Firestore

This change refactors the Firestore integration to support multiple databases within a single `FirebaseApp`, similar to the Node.js Admin SDK.

Key changes:
- `app.firestore()` now accepts an optional `databaseId` to get or create a named database instance.
- Database instances are cached per `databaseId` to ensure the same instance is returned on subsequent calls.
- The `Firestore` service wrapper now manages multiple `googleapis_firestore.Firestore` delegates, one for each database ID.
- Settings are now passed during initialization via `app.firestore(settings: ...)`, and re-initialization with different settings is prevented.
- Added comprehensive unit and integration tests for multi-database functionality, CRUD operations, and emulator support.
- Refactored `EmulatorClient` into the `googleapis_firestore` package and removed redundant exception handling code.

* docs: update README.md with Firestore usage examples and corrections

* chore: fix lint errors

* feat: enhance Firestore example with multi-database support and usage scenarios

* feat: add googleapis_firestore dependency to pubspec.yaml

* feat: add failedPrecondition error code and update error messages in Firestore

* chore: comment out local Firestore emulator configuration in coverage script

* refactor(firestore): simplify and improve firestore tests

Author: demolaf

packages/googleapis_firestore/lib/src/bulk_writer.dart

@@ -912,3 +912,4 @@ class BulkWriter {
     _bulkCommitBatch = _BulkCommitBatch(firestore, size);
   }
 }
+

packages/googleapis_firestore/lib/src/firestore.dart

@@ -14,6 +14,7 @@ import 'package:http/http.dart'
     show BaseRequest, StreamedResponse, ByteStream, BaseClient, Client;
 import 'package:intl/intl.dart';
 import 'package:meta/meta.dart';
+
 import 'backoff.dart';
 import 'environment.dart';
 

packages/googleapis_firestore/lib/src/rate_limiter.dart

@@ -115,3 +115,4 @@ class RateLimiter {
   @visibleForTesting
   int get maximumCapacity => _maximumCapacity;
 }
+