lib: json: add JSON_OBJ_DESCR_*_NAMED variants

The set of valid JSON field names is larger than the set of C
identifiers. This can result in structure field names which pack
decoded JSON values which necessarily differ from the field names in
the JSON.

Support this by adding _NAMED variants to each of the JSON_OBJ_DESCR_*
helper macros. For example, JSON_OBJ_DESCR_PRIM_NAMED allows users to
declare a descriptor field for a primitive type, whose structure field
name is different from the JSON field name.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>