Add ParagonIE Encoding dependency to composer.json.
requested to merge issue/tfa-3498141:3498141-paragonieconstanttimeencoding-dependency-not into 8.x-1.x
Closes #3498141 by fixing the following error:
Error: Class "ParagonIE\ConstantTime\Encoding" not found in Drupal\tfa\Plugin\TfaValidation\TfaTotpValidation->validate() (line 316 of modules/contrib/tfa/src/Plugin/TfaValidation/TfaTotpValidation.php).
This change adds 'paragonie/constant_time_encoding' as a dependency in TFA's composer.json.