Draft: Proto: implemented storage modules for the counter exercise

Context

The storage module for the global counter is implemented in this MR. The global counter supports two operations, reading the value and incrementing the value by 1.

This is the first step for the global counter exercise.

Manually testing the MR

Unit tests have been added for the global counter storage module. The test can be run with:

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe \
                  -- --file test_global_counter_storage.ml

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