[node] Refactor blob expiration key

[ian-shim]

Why are these changes needed?

Instead of representing blob expiration value as a slice of blob header hashes, make each expiration key encode blob header hash so it represents expiration for a single blob. Context: https://github.com/Layr-Labs/eigenda/pull/676#discussion_r1711801426

To be merged on top of https://github.com/Layr-Labs/eigenda/pull/676

Checks

• [ ] I've made sure the lint is passing in this PR.
• [ ] I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• Testing Strategy
  • [ ] Unit tests
  • [ ] Integration tests
  • [ ] This PR is not tested :(