Adding admin API implementation for modules

[Hrithik98]

Adding admin API implementation for modules API and corresponding tests.
The original implemenation for all modules APIs is as it is and the behavior can be toggled by using the environment variable MODULES_USE_ADMIN_API in app.yaml

when MODULES_USE_ADMIN_API is set to true, admin API implementation will be used.
The original implementation will be used in all other cases

• Tests pass

[ludoch]

Not sure if I am part of these reviews, I forgot.
One thing is instead of MODULES_USE_ADMIN_API variable name could you prepend it with APPENGINE_ to have a better namespace in envs? If other changes need env variables, we could try to also be consistent with the prefix.