json: Add get_map() method to JSON object class

json: Add get_map() method to JSON object class

This patch adds a get_map () method to the JSON object class to provide access
to the underlying hash map that stores the JSON key-value pairs.

To do this, we expose the map_t typedef, the return type of get_map().

This change is needed to allow traversal of key-value pairs when parsing
user-provided JSON tuning data.

Additionally, is_a_helper template specializations for json::literal * and
const json::literal * were added to make dynamic casting in the next patch
easier.

This patch was bootstrapped and regtested on aarch64-linux-gnu, no regression.

Signed-off-by: Soumya AR <soumyaa@nvidia.com>
gcc/ChangeLog:

	* json.h (class object): Add get_map () method.
	(is_a_helper<json::literal *>, is_a_helper<const json::literal *>):
	New template specializations.