Indicate feature toggle optional fields

This helps get the code annotations linter pass on edx-platform.

Author: regisb

code_annotations/base.py

@@ -445,8 +445,10 @@ def check_results(self, all_results):
 
     def iter_groups(self, annotations):
         """
-        Iterate on groups of annotations. Annotations are considered as a group when they all have the same
-        `line_number`, which should point to the beginning of the annotation group.
+        Iterate on groups of annotations.
+
+        Annotations are considered as a group when they all have the same `line_number`, which should point to the
+        beginning of the annotation group.
 
         Yield:
             annotations (annotation list)
@@ -468,7 +470,9 @@ def iter_groups(self, annotations):
 
     def check_group(self, annotations):
         """
-        Perform several linting checks on a group of annotations:
+        Perform several linting checks on a group of annotations.
+
+        The following checks are performed:
 
         - Choice fields should have a valid value
         - Annotation tokens are valid

code_annotations/contrib/config/feature_toggle_annotations.yaml

@@ -16,8 +16,11 @@ annotations:
         choices: [temporary, circuit_breaker, vip, opt_out, opt_in, open_edx]
     - ".. toggle_creation_date:":
     - ".. toggle_target_removal_date:":
+        optional: true
     - ".. toggle_warnings:":
+        optional: true
     - ".. toggle_tickets:":
+        optional: true
 extensions:
     python:
         - py