Support native_type for tables when using the C++ object API. (#8668)

* Support native_type for tables when using the C++ object API.

If native_type is specified on a table:
- No object API struct type is generated.
- The object API refers to the table by its native_type.
- UnPack and Create<TableName> methods are declared but not defined; as they
  must be user-provided.

* Add tests for native_type on tables.

* Add documentation for native_type on tables.

docs/source/languages/cpp.md

@@ -242,6 +242,41 @@ provide the following functions to aide in the serialization process:
     }
 ```
 
+-   `native_type("type")` (on a table): Tables can also be represented with
+native types. For example, the following schema:
+
+```cpp
+    table Matrix (native_type: "NativeMatrix") {
+      rows: int32;
+      columns: int32;
+      values: [float];
+    }
+```
+
+Would be represented by a user-defined C++ class:
+
+```cpp
+    class NativeMatrix { ... }
+```
+
+In this case, the following function declarations are generated by the compiler.
+The user must provide and link the matching function definitions:
+
+```cpp
+    struct Matrix FLATBUFFERS_FINAL_CLASS : private ::flatbuffers::Table {
+      // ...
+
+      static ::flatbuffers::Offset<Matrix> Pack(
+          ::flatbuffers::FlatBufferBuilder& _fbb,
+          const NativeMatrix* _o,
+          const ::flatbuffers::rehasher_function_t* _rehasher = nullptr);
+
+      void UnPackTo(
+          NativeMatrix* _o,
+          const ::flatbuffers::resolver_function_t* _resolver = nullptr) const;
+    }
+```
+
 Finally, the following top-level attributes:
 
 -   `native_include("path")` (at file level): Because the `native_type` attribute