Refactor FieldDef to model presense as an enum rather than 2 bools. (#6420)

* Define presence.

* Migrate to IsRequired and IsOptional methods

* moved stuff around

* Removed optional and required bools from FieldDef

* change assert to return error

* Fix tests.cpp

* MakeFieldPresence helper

* fmt

* old c++ compatibility stuff

Co-authored-by: Casper Neo <cneo@google.com>

src/idl_gen_json_schema.cpp

@@ -239,7 +239,7 @@ class JsonSchemaGenerator : public BaseGenerator {
       std::vector<FieldDef *> requiredProperties;
       std::copy_if(properties.begin(), properties.end(),
                    back_inserter(requiredProperties),
-                   [](FieldDef const *prop) { return prop->required; });
+                   [](FieldDef const *prop) { return prop->IsRequired(); });
       if (!requiredProperties.empty()) {
         auto required_string(Indent(3) + "\"required\" : [");
         for (auto req_prop = requiredProperties.cbegin();