Support Relay as Object Cache driver for Redis

[Relay](https://relay.so/) is "The next-generation caching layer for PHP." LSPHP recently started building it as an available extension. [Here's the implementation in the Redis Cache Plugin](https://github.com/rhubarbgroup/redis-cache/blob/abe85854aadbb9dccb831f4599e57405180a967f/includes/object-cache.php#L770).

Start by reading the linked Redis Cache Plugin implementation in includes/object-cache.php and comparing it with this repository's Redis object-cache code. Trace how the cache driver is selected, then verify that Relay can be supported as a Redis object-cache driver without breaking existing Redis behavior.

Written by the indexing model from the issue text.