feat(postgrest, supabase): add useSchema() method for making rest API calls on custom schema.

packages/postgrest/lib/src/postgrest.dart

@@ -61,6 +61,19 @@ class PostgrestClient {
     );
   }
 
+  /// Select a schema to query or perform an function (rpc) call.
+  ///
+  /// The schema needs to be on the list of exposed schemas inside Supabase.
+  PostgrestClient useSchema(String schema) {
+    return PostgrestClient(
+      url,
+      headers: {...headers},
+      schema: schema,
+      httpClient: httpClient,
+      isolate: _isolate,
+    );
+  }
+
   /// Perform a stored procedure call.
   ///
   /// ```dart

packages/postgrest/test/basic_test.dart

@@ -88,6 +88,19 @@ void main() {
       expect(res.length, 5);
     });
 
+    test('query non-public schema dynamically', () async {
+      final postgrest = PostgrestClient(rootUrl);
+      final personalData = await postgrest
+          .useSchema('personal')
+          .from('users')
+          .select<PostgrestList>();
+      expect(personalData.length, 5);
+
+      // confirm that the client defaults to its initialized schema by default.
+      final publicData = await postgrest.from('users').select<PostgrestList>();
+      expect(publicData.length, 4);
+    });
+
     test('on_conflict upsert', () async {
       final res = await postgrest.from('users').upsert(
         {'username': 'dragarcia', 'status': 'OFFLINE'},

packages/supabase/lib/src/supabase_client.dart

@@ -180,6 +180,13 @@ class SupabaseClient {
     );
   }
 
+  /// Select a schema to query or perform an function (rpc) call.
+  ///
+  /// The schema needs to be on the list of exposed schemas inside Supabase.
+  PostgrestClient useSchema(String schema) {
+    return rest.useSchema(schema);
+  }
+
   /// Perform a stored procedure call.
   PostgrestFilterBuilder rpc(
     String fn, {