[functions] build_runner integration

[dackers86]

Implement build_runner integration for the Dart Firebase Functions SDK within firebase-tools. This includes:

• Code generation setup using Dart's build_runner
• Generation of functions.yaml specification file in .dart_tool/firebase/ directory
• Automatic watching for code changes to rebuild the specification file
• Removing the need for a discovery-phase HTTP server

This allows use of emulator building through the Firebase CLI.

Local Development:
To test locally, use npm link in the firebase-tools repository:

1. Clone firebase-tools repository
2. Run npm install in firebase-tools
3. Run npm link to create global symlink
4. In your test project, run npm link firebase-tools

Dependencies:

• [functions] Repository setup
• [functions] SDK scaffolding

Acceptance Criteria:

• build_runner package configured
• Code generation for functions.yaml implemented
• File output to .dart_tool/firebase/ directory working
• Watch mode for automatic rebuilds functional
• Local development instructions documented