Add optional flag to message description

[iam047801]

Some messages might have such fields, that users can omit them and do no put in the payload. For example, it can be forward_payload in nft_item_transfer message, or custom_payload in jetton_burn.

If field does not exists in the payload, Anton fails with the following error:
{"level":"warn","error":"load from cell: failed to load maybe for CustomPayload, err: not enough data in reader, need 1, has 0","tx_hash":"8ad6febf40eed096d8d911466c069ebc693e4d5e641a4e806a19d5831233141b","dst_addr":"EQAszkABaoiX_-JnjFXSo39Ryoept-9-W2Pmd3Y2mO4F0bHb","op_id":1499400124,"time":1684256426,"caller":"/go/src/github.com/tonindexer/anton/internal/app/parser/tx.go:90","message":"parse dst [jetton_wallet] message"}

We should skip such fields, just parse other fields and do not fail.