Disable external initializers build option (#7635)
* Merge set custom allocator to master
* Add documentation for the new API.
Reset global env in testCustomArenaAllocator so won't have a registered allocator of type arena (from previous test)
* Add a session option config that will allow to disable loading model with initializers that have an external data (+test it).
* Add the model used for the test and its external initializers data
* Change the session config option that disable external initializers to a build option.
* Addressing PR comments
