feat: GetAsk API

[LexLuthr]

This PR recreates the legacy MarketGetAsk API as it is required by the CIDGravity team. The API was removed as part of the go-fil-market removal.

% curl -s -X POST -H "Content-Type: application/json" --data '{ "jsonrpc": "2.0", "method": "Filecoin.MarketGetAsk", "params": [], "id": 1}' http://localhost:1288/rpc/v0 | jq
{
  "jsonrpc": "2.0",
  "result": {
    "Ask": {
      "Price": "50000000",
      "VerifiedPrice": "5000000",
      "MinPieceSize": 256,
      "MaxPieceSize": 34359738368,
      "Miner": "t01000",
      "Timestamp": 25,
      "Expiry": 1000025,
      "SeqNo": 0
    },
    "Signature": {
      "Type": 2,
      "Data": "lylEWYA/AH1+LXfknJai5sZOuCOFqHChWCTVkPrELx+uze7h5dv2TbGWZtGsRfEkEJ44Bo32EWR8MdNs/oGQSXcqKt/PTVBgRsze2yjbiRI+L74dCWFTpYgOSG+3PY7M"
    }
  },
  "id": 1
}

[Normalnoise]

supported