etherlink: switch private to public when file exists.

Context

At stage 0 if there is a value in the storage at /evm/private_rollup then submit a outbox message to make the rollup public.

previous: !12903 (closed)

Manually testing the MR

run the given test teztether --file evm_rollup.ml alpha whitelist --verbose or do manually the step to make sure it works.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR