BE 1.5 - Implement get_volunteers_table function

[LeandroHamaguchi]

Implement the get_volunteers_table function to fetch all the volunteers' data from the database.

Return a list of volunteer objects. Use types declared in src/lib/client/supabase/types.ts for more precise return values. Example:

[
{ volunteer: Volunteer1, cohorts: [Cohort1Volunteer1, Cohort2Volunteer1], roles: [Role1Volunteer1, Role2Volunteer1] },
{ volunteer: Volunteer2, cohorts: [Cohort1Volunteer2, Cohort2Volunteer2, Cohort3Volunteer2], roles: [Role1Volunteer2] },
...
]

Implement tests for this function.