feature(tarantool): add nullable fields in schema module

tarantool/src/schema/space.rs

@@ -125,6 +125,7 @@ fn insert_new_space(
             let mut field_format = Map::<String, Value>::new();
             field_format.insert("name".to_string(), Value::String(ft.name.clone()));
             field_format.insert("type".to_string(), Value::String(ft.field_type.to_string()));
+            field_format.insert("is_nullable".to_string(), Value::Bool(ft.is_nullable));
             space_format.push(Value::Object(field_format));
         }
     }

tarantool/src/space.rs

@@ -149,6 +149,7 @@ pub struct SpaceFieldFormat {
     pub name: String,
     #[serde(alias = "type")]
     pub field_type: SpaceFieldType,
+    pub is_nullable: bool,
 }
 
 impl SpaceFieldFormat {
@@ -156,6 +157,7 @@ impl SpaceFieldFormat {
         SpaceFieldFormat {
             name: name.to_string(),
             field_type: ft,
+            is_nullable: false,
         }
     }
 }