[functions] SDK scaffolding

[dackers86]

Set up the initial project scaffolding for the Dart Firebase Functions SDK. This includes:

• Project directory structure
• Package configuration (pubspec.yaml)
• Base exports and module organization

This provides the physical structure upon which all function triggers and features will be built.

Dependencies:

• [functions] Repository setup

Acceptance Criteria:

• Project directory structure created
• Package configuration (pubspec.yaml) set up
• Base exports and module organization established
• Initial README with project overview