Add ParagonIE Encoding dependency to composer.json.

(Potentially) closes #2922287.

As per comment #11 on the abovementioned issue, we were experiencing the following error on our D10.3.10 site with TFA on 8.x-1.9:

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).

We fixed it by requiring 'paragonie/constant_time_encoding' on the whole project, but really it seems to me from reading through this issue and the other related issues sprouting from/to it (principally TFA issue #2866841 and Google Authenticator issue #2880601) that said dependency should be brought into the codebase just by TFA itself, hence adding it here to the composer.json. I've tested it locally and it works, though I accept there may be some reason it was excluded or some other better solution - please do let me know if so and many thanks in advance.