fix(firebase): validate custom serverFunctionName

[jpsc]

🔗 Linked issue

Resolves #2643

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Normalize serverFunctionName to using underscore (_) instead of dashes (-).

Resolves #2643

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[jpsc]

Not sure how to properly validade this fix.

[pi0]

Thank you!