feat(op-celestia/indexer): add indexed l2 block status

[fakedev9999]

Adds l2 start and end block of the indexer for better qol.

Response of admin_getIndexerStatus is updated as below.

taehoon@succinct-gpu-06:~/celestia-optimism/op-celestia$ curl -X POST -H "Content-Type: application/json" -s \
  --data '{"jsonrpc":"2.0","method":"admin_getIndexerStatus","params":[],"id":1}' \
  http://localhost:8545 | jq .
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "last_indexed_block": 9076129,
    "indexed_blocks": 14148,
    "running": true,
    "l2_start_block": 26263190,
    "l2_end_block": 26277337
  }
}

[tuxcanfly]

OK.