Expose Convert.decodeHex(hex: string) function

[dgarcia360]

1. Description of the problem
When updating a metadata entry, we can use the MetadataTransactionService. This service xors the current and new metadata values to get the difference between both strings.

However, if we try to update a metadata entry announcing a regular MetadataTransaction (without using the MetadataTransactionService ), we need to use as well the function Convert.decodeHex(hex: string) after xor-ing the result. The above-mentioned function is defined as @internal and can't be used.

2. Steps to reproduce the problem

Try to use Convert.decodeHex(Convert.xor(currentValueBytes, newValueBytes)) (0.14.3)