BE 1 - Implement create_volunteer API function

[notjackl3]

Create an API function to add volunteers into the database. Here are some resources:

• Our Supabase dashboard's documentation on how to work with Volunteers table
• Our Supabase Volunteer table schema.

Notes:

• The API functions should be written inside src/lib/api/
• Ensure proper error handling and validation of input data
• If you need to install any dependency (unlikely), write about it on your PR.

The goal is to:

• Create a POST endpoint that accepts volunteer data and inserts it into the Supabase database
• Return appropriate success/error responses with HTTP status codes.