Implements support for the new /eth/v1/beacon/blobs/{block_id} endpoint
with automatic fallback to the legacy blob_sidecars endpoint.

Key changes:
- Archiver: Prefer blobs endpoint, compute KZG commitments/proofs from
  blob data, fall back to blob_sidecars on failure
- API: Add /eth/v1/beacon/blobs/{id} endpoint with versioned_hashes
  filtering support using kzg4844.CalcBlobHashV1
- Test data: Generate valid BLS12-381 field elements by reducing random
  data modulo the field modulus to ensure KZG operations succeed
- Test helpers: Consolidate blob generation into NewBlobSidecars with
  proper header support for consistent test comparisons

All tests now use the blobs endpoint (except one that explicitly tests
the fallback behavior). The archiver stores complete blob sidecars with
derived KZG commitments and proofs, maintaining backward compatibility
with existing storage format.

Technical notes:
- Each 32-byte chunk in blob data must be < BLS12-381 field modulus
- KZG commitments/proofs are computed using gokzg4844 library
- Versioned hashes use ethereum/go-ethereum's kzg4844.CalcBlobHashV1
- Test data includes valid headers to match reconstructed sidecars

Switch to prioritizing /eth/v1/beacon/blob_sidecars over /eth/v1/beacon/blobs
since the sidecars endpoint provides KZG commitments and proofs directly,
avoiding unnecessary computation.

The go-kzg-4844 package is now directly used in the codebase after
migrating to the beacon blobs endpoint, so it should be listed as a
direct dependency rather than an indirect one.

Replaced custom field modulus constant with gokzg4844.BlsModulus, which
is the canonical BLS12-381 scalar field modulus from our existing
dependency. This fixes an incorrect modulus value and provides a single
source of truth for the BLS_MODULUS constant as specified in EIP-4844.

The go-kzg-4844 library already exports this value as BlsModulus, so
there's no need to duplicate it.