feat: add function to check if username is taken

Signed-off-by: Raphael Arce <raphael.arce@ts.berlin>

Author: raphael-arce

supabase/migrations/20241011145739_add_is_username_taken_function.sql

@@ -0,0 +1,14 @@
+CREATE OR REPLACE FUNCTION is_username_taken(given_username text)
+RETURNS BOOLEAN AS $$
+DECLARE
+    is_username_taken BOOLEAN;
+BEGIN
+    -- Check if the username exists in the profile table
+    SELECT EXISTS(SELECT 1 FROM public.profiles WHERE username = given_username) INTO is_username_taken;
+    
+    -- Return the result
+    RETURN is_username_taken;
+END;
+$$
+LANGUAGE plpgsql
+SECURITY DEFINER;