gen-object-api for dictionaries/maps #8399

As far as I'm aware, --gen-object-api does not currently generate native map containers such as std::unordered_map or std::map for keyed vectors. Even when a table has a (key) field, the generated Object API still represents the data as a vector/container of objects.

I suspect this is intentional for a few reasons:

• The FlatBuffers schema defines a vector of tables, not a map type.
• The (key) attribute primarily enables efficient lookup operations (e.g. binary search via generated accessors) rather than changing the underlying data model.
• Generating language-specific map types would introduce additional complexity and potentially inconsistent behavior across supported languages.

For example, a schema like:

table Item {
  id:string (key);
  value:int;
}

table Container {
  items:[Item];
}

still fundamentally describes an ordered collection of Item objects rather than a true dictionary type.

That said, I can definitely see the appeal of having the Object API expose keyed vectors as std::unordered_map<Key, T> or similar. For applications that primarily perform lookups by key, that representation would often be more natural and could avoid repeatedly building lookup structures on top of the generated objects.

I don't believe there is currently a flag to force this behavior. If maintainers have considered it before, I'd be interested to know whether the main concerns are API compatibility, cross-language consistency, serialization semantics, or something else.