feat: add Functions Task Queue API with emulator support and tests

[demolaf]

Changes

• Added Task Queue admin API (Functions, TaskQueue) with enqueue/delete, scheduling, deadlines, headers, custom task IDs, and optional custom URI.
• Added Functions getter to FirebaseApp and registered the service in FirebaseServiceType.
• Added Cloud Tasks emulator support via CLOUD_TASKS_EMULATOR_HOST

Examples

• Added emulator config + example Functions TS app and a Dart example for task queue usage.

Testing

• Added extensive unit tests and emulator integration tests for Task Queue behavior.
• Updated coverage script to build test functions and run tasks/functions emulators.

[github-actions]

Coverage Report

✅ Coverage 73.36% meets 40% threshold

Coverage: 73.36%
Lines Covered: 3790/5166

Minimum threshold: 40%