[GraphQL] Add dossierEnvoyerMessage mutation

app/graphql/schema.graphql

@@ -352,6 +352,33 @@ type DossierEdge {
   node: Dossier
 }
 
+"""
+Autogenerated input type of DossierEnvoyerMessage
+"""
+input DossierEnvoyerMessageInput {
+  attachment: ID
+  body: String!
+
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+  dossierId: ID!
+  instructeurId: ID!
+}
+
+"""
+Autogenerated return type of DossierEnvoyerMessage
+"""
+type DossierEnvoyerMessagePayload {
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+  errors: [ValidationError!]
+  message: Message
+}
+
 type DossierLinkChamp implements Champ {
   dossier: Dossier
   id: ID!
@@ -495,6 +522,11 @@ type Mutation {
   File information required to prepare a direct upload
   """
   createDirectUpload(input: CreateDirectUploadInput!): CreateDirectUploadPayload
+
+  """
+  Envoyer un message à l'usager du dossier.
+  """
+  dossierEnvoyerMessage(input: DossierEnvoyerMessageInput!): DossierEnvoyerMessagePayload
 }
 
 enum Order {
@@ -806,4 +838,14 @@ enum TypeDeChamp {
 """
 A valid URL, transported as a string
 """
-scalar URL
+scalar URL
+
+"""
+Éreur de validation
+"""
+type ValidationError {
+  """
+  A description of the error
+  """
+  message: String!
+}