Add block signatures in beacon header RPC endpoints

[fridrik01]

Issue #2992

Context

Block signatures were missing from the beacon header RPC endpoints (/eth/v1/beacon/headers and /eth/v1/beacon/headers/{block_id}), returning empty strings instead of the actual BLS signatures. This was because the block store only persisted block metadata (roots, timestamps) but not signatures.

This PR extends the in-memory block store to also cache block signatures during finalization, and updates the header handler to retrieve and include them in API responses.

Signatures are available for blocks within the availability window. For genesis or blocks outside this window, an empty string is returned (like its currenltly).

Test plan

> make start
> make start-geth
> curl -s http://localhost:3500/eth/v1/beacon/headers | jq
{
  "execution_optimistic": false,
  "finalized": true,
  "data": [
    {
      "root": "0xbd6a1cef0ce2f3bc49cefcb4ea1e73ba551005e5d9b16218958534e0729b8178",
      "canonical": true,
      "header": {
        "message": {
          "slot": "4",
          "proposer_index": "0",
          "parent_root": "0x39218bb0854655d64eb60167b470fa0047a941cf5b8f2ee54c85e775a73653fa",
          "state_root": "0xcc26439d46b0eed3b7ca3e1bea7dea71b2393d2581d27ad6bc8009c0b22e5cec",
          "body_root": "0xbd6a1cef0ce2f3bc49cefcb4ea1e73ba551005e5d9b16218958534e0729b8178"
        },
        "signature": "0xa2db3b4c2cd220eb8ecdaf047514c100d09ea63560925efa88004f9a820d5aaa057b658d1b43583b2d0aedeb63dc9f15185edd1b9cde1205a2395a92575fee5142f03fc40bb
241b045f6b97d9682a0f2e65ec5efc7ccae8ba5093305b16140f9"
      }
    }
  ]
}

On a restart, querying the slot 4 again results in a null signature (since this PR uses in-memory store)

> curl -s http://localhost:3500/eth/v1/beacon/headers/4 | jq
{
  "execution_optimistic": false,
  "finalized": true,
  "data": {
    "root": "0xbd6a1cef0ce2f3bc49cefcb4ea1e73ba551005e5d9b16218958534e0729b8178",
    "canonical": true,
    "header": {
      "message": {
        "slot": "4",
        "proposer_index": "0",
        "parent_root": "0x39218bb0854655d64eb60167b470fa0047a941cf5b8f2ee54c85e775a73653fa",
        "state_root": "0xcc26439d46b0eed3b7ca3e1bea7dea71b2393d2581d27ad6bc8009c0b22e5cec",
        "body_root": "0xbd6a1cef0ce2f3bc49cefcb4ea1e73ba551005e5d9b16218958534e0729b8178"
      },
      "signature": ""
    }
  }
}

[codecov]

Codecov Report

❌ Patch coverage is 35.48387% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.46%. Comparing base (44333a9) to head (b7ee7e0).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
storage/block/store.go	21.42%	10 Missing and 1 partial ⚠️
node-api/handlers/beacon/header.go	50.00%	4 Missing and 1 partial ⚠️
node-api/backend/getters.go	0.00%	2 Missing ⚠️
beacon/blockchain/finalize_block.go	75.00%	0 Missing and 1 partial ⚠️
consensus/cometbft/service/finalize_block.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2997      +/-   ##
==========================================
- Coverage   62.54%   62.46%   -0.08%     
==========================================
  Files         353      353              
  Lines       17099    17117      +18     
==========================================
- Hits        10694    10692       -2     
- Misses       5558     5576      +18     
- Partials      847      849       +2     

Files with missing lines	Coverage Δ
beacon/blockchain/finalize_block.go	65.18% <75.00%> (-0.26%)	⬇️
consensus/cometbft/service/finalize_block.go	51.32% <0.00%> (ø)
node-api/backend/getters.go	35.82% <0.00%> (-1.11%)	⬇️
node-api/handlers/beacon/header.go	75.00% <50.00%> (-4.55%)	⬇️
storage/block/store.go	69.33% <21.42%> (-11.32%)	⬇️

... and 1 file with indirect coverage changes

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