Expose epic participants on GraphQL

Expose field participants for epics on GraphQL

ee/app/graphql/types/epic_type.rb

@@ -57,6 +57,7 @@ module Types
     field :reference, GraphQL::STRING_TYPE, null: false, method: :epic_reference do # rubocop:disable Graphql/Descriptions
       argument :full, GraphQL::BOOLEAN_TYPE, required: false, default_value: false # rubocop:disable Graphql/Descriptions
     end
+    field :participants, Types::UserType.connection_type, null: true, complexity: 5, description: 'List of participants for the epic'
 
     field :subscribed, GraphQL::BOOLEAN_TYPE,
       method: :subscribed?,

ee/changelogs/unreleased/issue_31920.yml

+---
+title: Expose epic participants on GraphQL
+merge_request: 18691
+author:
+type: added

ee/spec/graphql/types/epic_type_spec.rb

@@ -9,8 +9,8 @@ describe GitlabSchema.types['Epic'] do
       start_date start_date_is_fixed start_date_fixed start_date_from_milestones
       due_date due_date_is_fixed due_date_fixed due_date_from_milestones
       closed_at created_at updated_at children has_children has_issues
-      web_path web_url relation_path reference issues
-      user_permissions notes discussions relative_position subscribed
+      web_path web_url relation_path reference issues user_permissions
+      notes discussions relative_position subscribed participants
     ]
   end
 
@@ -23,4 +23,6 @@ describe GitlabSchema.types['Epic'] do
   it { expect(described_class).to have_graphql_fields(fields) }
 
   it { is_expected.to have_graphql_field(:subscribed, complexity: 5) }
+
+  it { is_expected.to have_graphql_field(:participants, complexity: 5) }
 end