[App] Add env variables to desactivate pull and push of the App State

[tchaton]

What does this PR do?

This PR introduces 2 env variables to disable state manipulation through the Rest API:

ENABLE_PULLING_STATE_ENDPOINT=0 # Used to disable getting the latest state
ENABLE_PUSHING_STATE_ENDPOINT=0  # Used to disable pushing deltas through the Rest API

Furthermore, this PR adds a layout endpoint to remove the need for the UI to fetch the AppState.

Fixes #<issue_number>

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