Store: Add option to decode cemented file for 64 bit offsets

What

Closes #7034 (closed)

Why

How

We introduce:

• the ability to read cemented files which have 32-bit or 64-bit offsets (we can determine which format we deal with in the code with a simple calculation on bits)
• a new behaviour for the "cements blocks" function, which now always writes cemented files with 64-bit offsets

Manually testing the MR

CI green +

$ dune exec src/lib_store/unix/test/main.exe -- --file test_cemented_store.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