feat: allow distinguishing between null and absent values in gql_code_builder (#381)

* feat: allow distinguishing between null and absent values

* pubspec clean

* format

* feat(gql_code_builder): make tri-state optional configurable, and end to end test with tri state null

* docs(gql_build): add documentation on tristate_optionals

* refactor(gql_code_builder): re-organize code to avoid cyclical deps

* fmt

* refactor: somewhat extract the logic for the value class implementation

* fix: add url to builtvaluefield ref check

* bump versions, update CHANGELOG

Author: knaeckeKami

.github/workflows/dart.yml

@@ -125,6 +125,7 @@ jobs:
       matrix:
         package:
           - end_to_end_test
+          - end_to_end_test_tristate
     runs-on: ubuntu-latest
     container:
       image: dart

codegen/end_to_end_test/lib/graphql/__generated__/schema.ast.gql.dart

@@ -24,6 +24,7 @@ type Query {
 # The mutation type, represents all updates we can make to our data
 type Mutation {
   createReview(episode: Episode, review: ReviewInput!, createdAt: Date): Review
+  createCustomField(input: CustomFieldInput!): CustomField
 }
 
 # The subscription type, represents all subscriptions we can make to our data
@@ -158,6 +159,11 @@ input ReviewInput {
   seenOn: [Date]
 }
 
+input CustomFieldInput {
+  id: ID!
+  customField: CustomField
+}
+
 # The input object sent when passing in a color
 input ColorInput {
   red: Int!