Warn when wp transient list is used with external object cache

[danielbachhuber]

Fixes #61

$ wp transient list
+---------------------------------+------------------------------------------+------------+
| name                            | value                                    | expiration |
+---------------------------------+------------------------------------------+------------+
| doing_cron                      | 1677266261.8137209415435791015625        | false      |
| health-check-site-status-result | {"good":16,"recommended":4,"critical":1} | false      |
+---------------------------------+------------------------------------------+------------+
$ wp plugin install --activate wp-redis
Installing WP Redis (1.3.2)
Downloading installation package from https://downloads.wordpress.org/plugin/wp-redis.1.3.2.zip...
The authenticity of wp-redis.1.3.2.zip could not be verified as no signature was found.
Unpacking the package...
Installing the plugin...
Plugin installed successfully.
Activating 'wp-redis'...
Plugin 'wp-redis' activated.
Success: Installed 1 of 1 plugins.
$ wp redis enable
Success: Enabled WP Redis by creating wp-content/object-cache.php symlink.
$ wp transient list
Warning: Transients are stored in an external object cache, and this command only shows those stored in the database.
+---------------------------------+------------------------------------------+------------+
| name                            | value                                    | expiration |
+---------------------------------+------------------------------------------+------------+
| health-check-site-status-result | {"good":16,"recommended":4,"critical":1} | false      |
+---------------------------------+------------------------------------------+------------+