Implement the support to t:sqlHexDecode into libmodsecurity

[zimmerle]

Transformation sqlHexDecode needs to be implemented inside libmodsecurity.

The documentation of the transformation can be found here:
https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#sqlHexDecode

The file already exist in the repository:
https://github.com/SpiderLabs/ModSecurity/blob/libmodsecurity/src/actions/transformations/sql_hex_decode.cc