Implement scalar functions: abs, upper, lower, reverse (#61)

* Implement scalar functions: abs, upper, lower, reverse

Closes #32

* Update cratedb_django/models/functions.py

Co-authored-by: Ivan <surister98@gmail.com>

---------

Co-authored-by: Ivan <surister98@gmail.com>

Author: talhakiriiss-cyber

cratedb_django/models/functions.py

@@ -10,3 +10,28 @@ class UUID(Func):
     output_field = TextField(
         max_length=20
     )  # the length of a CrateDB random uid.
+
+class Abs(Func):
+    """
+    https://cratedb.com/docs/crate/reference/en/latest/general/builtins/scalar-functions.html#abs
+    """
+    function = "abs"
+
+class Upper(Func):
+    """
+    https://cratedb.com/docs/crate/reference/en/latest/general/builtins/scalar-functions.html#upper
+    """
+    function = "upper"
+
+class Lower(Func):
+    """
+    https://cratedb.com/docs/crate/reference/en/latest/general/builtins/scalar-functions.html#lower
+    """
+    function = "lower"
+
+class Reverse(Func):
+    """
+    https://cratedb.com/docs/crate/reference/en/latest/general/builtins/scalar-functions.html#reverse
+    """
+    function = "reverse"
+