Refs #28643 -- Added LTrim, RTrim, and Trim database functions.

Thanks Tim Graham and Mads Jensen for reviews.

tests/db_functions/test_trim.py

@@ -0,0 +1,40 @@
+from django.db.models import CharField
+from django.db.models.functions import LTrim, RTrim, Trim
+from django.test import TestCase
+
+from .models import Author
+
+
+class TrimTests(TestCase):
+    def test_trim(self):
+        Author.objects.create(name='  John ', alias='j')
+        Author.objects.create(name='Rhonda', alias='r')
+        authors = Author.objects.annotate(
+            ltrim=LTrim('name'),
+            rtrim=RTrim('name'),
+            trim=Trim('name'),
+        )
+        self.assertQuerysetEqual(
+            authors.order_by('alias'), [
+                ('John ', '  John', 'John'),
+                ('Rhonda', 'Rhonda', 'Rhonda'),
+            ],
+            lambda a: (a.ltrim, a.rtrim, a.trim)
+        )
+
+    def test_trim_transform(self):
+        Author.objects.create(name=' John  ')
+        Author.objects.create(name='Rhonda')
+        tests = (
+            (LTrim, 'John  '),
+            (RTrim, ' John'),
+            (Trim, 'John'),
+        )
+        for transform, trimmed_name in tests:
+            with self.subTest(transform=transform):
+                try:
+                    CharField.register_lookup(transform)
+                    authors = Author.objects.filter(**{'name__%s' % transform.lookup_name: trimmed_name})
+                    self.assertQuerysetEqual(authors, [' John  '], lambda a: a.name)
+                finally:
+                    CharField._unregister_lookup(transform)