BE 1.5 - Implement filter_by_cohort API function

[notjackl3]

Create an API function to filter volunteers by their cohorts. Here are some resources:

• Our Supabase dashboard's documentation on how to work with Volunteers table
• Our Supabase Volunteer table schema.
• Our Supabase dashboard's documentation on how to work with VolunteerCohorts table
• Our Supabase VolunteerCohorts table schema.
• Our Supabase dashboard's documentation on how to work with Cohorts table
• Our Supabase Cohorts table schema.
• Our database schema which helps you visualize how the tables are connected

Params:

• < op>: string - Either "AND" [to require matching all values] or "OR" [to require matching any of the values])
• < values>: List<Tuple<string, string>> - the values to filter for (e.g. [("Fall", "2025"), ("Spring", "2023")])

Returns:

• the resulting rows after filtering

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, write about it on your PR.

The goal is to:

• Create a GET endpoint that accepts role filter parameters and returns filtered volunteer data from Supabase
• Support filtering by single or multiple cohorts
• Return appropriate success/error responses with HTTP status codes.