Fix inconsistency in --fxid field usage

As discussed here, the field --fxid of the cta-admin tool should only accept an hexadecimal value:

• https://gitlab.cern.ch/cta/operations/-/issues/1594#note_9002890

The same applies to any other cmd line tool belonging to the CTA project.

For file IDs represented by a string (which can be their decimal value or a UUID), a new --diskfileid <string> parameter should be used.