chore: add sepolia timestamp asserter address

[MexicanAce]

Description

Add Sepolia TimestampAsserter address. Here's the contract in block explorer: https://sepolia.explorer.zksync.io/address/0xa64EC71Ee812ac62923c85cf0796aA58573c4Cf3#contract

Here's also the validation that it's deployed to the Sepolia API server:

curl --request POST \
  --url https://sepolia.era.zksync.dev/ \
  --header 'Content-Type: application/json' \
  --data '{
      "jsonrpc": "2.0",
      "id": 1,
      "method": "zks_getTimestampAsserter",
      "params": []
    }'

Response:

{"jsonrpc":"2.0","result":"0xa64ec71ee812ac62923c85cf0796aa58573c4cf3","id":1}

[ly0va]

LGTM