Add custom hex2bin implementation for 5.3

Fixes https://github.com/owncloud/core/issues/11004
Merge pull request #11006 from owncloud/addCustomHex2BinImplementationBecauseSupporting53IsSomethingReallyReallyCoolAndWeAreObviouslySomeOfTheCoolGuys

Add custom hex2bin implementation for 5.3