Add sms sender to service to be used in sms templates

in place of default numeric short code.

If not present default short code is used.

app/schemas.py

@@ -1,3 +1,4 @@
+import re
 from datetime import (
     datetime,
     date
@@ -106,6 +107,11 @@ class ServiceSchema(BaseSchema):
         exclude = ("updated_at", "created_at", "api_keys", "templates", "jobs", 'old_id')
         strict = True
 
+    @validates('sms_sender')
+    def validate_sms_sender(self, value):
+        if value and not re.match('^[a-zA-Z0-9\s]+$', value):
+            raise ValidationError('Only alphanumeric characters allowed')
+
 
 class NotificationModelSchema(BaseSchema):
     class Meta: