Add DECODE function to create BLOBs

[simoc]

Add a DECODE function like in PostreSQL to convert a hexadecimal or base 64 text string to a BLOB data type.

For example,

DECODE('FF7F00FC', 'hex')

DECODE('aGVsbG8=', 'base64')

This makes it possible to create a CSV file with test data containing BLOB columns.

Documentation for PostreSQL DECODE function is at
https://www.postgresql.org/docs/current/functions-binarystring.html