Draft: Shell: Improve attestations encoding for storage

Context

WIP for Optimise attestations encoding project.

Current issues:

• running on mainnet -> cannot run a node after importing a rolling snapshot:

octez-node: Error:
              Cannot find block at distance 0 from block BMaJ2yZhHs5BZuTXsnnVKHWALKfstRLqeUPzpgqRYmbCwChsuTu.

Manually testing the MR

TBD

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