Adding support to read an "external_subdirectories" key from the

project.json when building a project.
This allows the project to build additional gems via adding the external
subdirectory in the project.json file manually
or using the `o3de.py register --external-subdirectory-path=<path>
--external_subdirectory-project-path-<project-path>` command

Templates/DefaultGem/Template/CMakeLists.txt

@@ -11,7 +11,7 @@
 
 set(o3de_gem_path ${CMAKE_CURRENT_LIST_DIR})
 set(o3de_gem_json ${o3de_gem_path}/gem.json)
-o3de_gem_name(${o3de_gem_json} o3de_gem_name)
+o3de_read_json_key(o3de_gem_name ${o3de_gem_json} "gem_name")
 o3de_restricted_path(${o3de_gem_json} o3de_gem_restricted_path)
 
 # Currently we are in the DefaultProjectSource folder: ${CMAKE_CURRENT_LIST_DIR}