Expose read-only getter methods for configured cache expiration and janitor cleanup intervals on cache and shardedCache:

• DefaultExpiration() time.Duration: returns the default expiration duration configured for items.
• CleanupInterval() time.Duration: returns the interval duration between janitor cleanups (or 0 if the janitor is not running).

Both methods are automatically available on Cache and unexportedShardedCache via struct embedding.

Fixes #104