explode expects string, int given in TokenHooks.php

Problem/Motivation

When using devel module to view tokens on an entity containing a custom field (path /devel/token/[entity_type]/[entity_id]), I see

TypeError: explode(): Argument #2 ($string) must be of type string, int given in explode() (line 433 of modules/composer/custom_field/src/Hook/TokenHooks.php).

The argument in question is an array key retrieved with findWithPrefix(). In the event that a token is of the format [entity:some_custom_field:0], the key would be "0", which PHP silently changes to 0, breaking the explode() call.

Proposed resolution

Cast the key as a string when calling explode().

Remaining tasks

Make a MR.