Parsing from proto should keep field ID. (fixes #7645) (#7655)

* Parsing from proto should keep field ID. (fixes #7645)

* Fix failed tests

* Fix windows warning

* Improve attribute generation in proto to fbs

* Check if id is used twice. fix Some clang-format problems

* Test if fake id can solve the test problem

* Validate proto file in proto -> fbs generation.

* Fix error messages

* Ignore id in union

* Add keep proto id for legacy and check gap flag have been added. Reserved id will be checked.

* Add needed flags

* unit tests

* fix fromat problem. fix comments and error messages.

* clear

* More unit tests

* Fix windows build

* Fix include problems

* Fake commit to invoke rebuild

* Fix buzel build

* Fix some issues

* Fix comments, fix return value and sort for android NDK

* Fix return type

* Break down big function

* Place todo

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

include/flatbuffers/idl.h

@@ -381,6 +381,7 @@ struct StructDef : public Definition {
   size_t bytesize;  // Size if fixed.
 
   flatbuffers::unique_ptr<std::string> original_location;
+  std::vector<voffset_t> reserved_ids;
 };
 
 struct EnumDef;
@@ -594,7 +595,7 @@ inline bool operator<(const IncludedFile &a, const IncludedFile &b) {
 struct IDLOptions {
   // field case style options for C++
   enum CaseStyle { CaseStyle_Unchanged = 0, CaseStyle_Upper, CaseStyle_Lower };
-
+  enum class ProtoIdGapAction { NO_OP, WARNING, ERROR };
   bool gen_jvmstatic;
   // Use flexbuffers instead for binary and text generation
   bool use_flexbuffers;
@@ -663,6 +664,8 @@ struct IDLOptions {
   bool ts_no_import_ext;
   bool no_leak_private_annotations;
   bool require_json_eof;
+  bool keep_proto_id;
+  ProtoIdGapAction proto_id_gap_action;
 
   // Possible options for the more general generator below.
   enum Language {
@@ -769,6 +772,8 @@ struct IDLOptions {
         ts_no_import_ext(false),
         no_leak_private_annotations(false),
         require_json_eof(true),
+        keep_proto_id(false),
+        proto_id_gap_action(ProtoIdGapAction::WARNING),
         mini_reflect(IDLOptions::kNone),
         require_explicit_ids(false),
         rust_serialize(false),