Use first 16 bytes of xUDT cell data as an amount

[duanyytop]

Now the RGB++ SDK uses the whole xUDT cell data as an amount suitable for most scenarios. The xUDT protocol supports cell data with a length greater than 16 bytes.

See: https://blog.cryptape.com/enhance-sudts-programmability-with-xudt#heading-xudt-data-structures

To ensure the accuracy of xUDT cell data, the RGB++ SDK should cut the first 16 bytes as the xUDT amount.