Alpha/DAL: introduce Dal_attestations_repr

What

Add support for representing multiple attestations. (There will be one for each lag in attestation_lags, though this aspect is not handed in this MR).

Why

Needed by the dynamic lag approach for DAL latency.

Summary by CodeRabbit

• New Features

  • Efficient representation and management of multiple data-availability layer attestations.
  • Query functions to check if attestations are empty and whether specific slots are attested.
  • Encoding support for serializing attestations and size/occupancy utilities.

• Tests

  • Added comprehensive unit tests covering encoding, updates, edge cases, idempotency, and complex interaction patterns.

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