[10.x] Add function to validate if a lock is owned by the current owner.

[Joostb]

This pr adds a public function isOwner to the Cache/Lock.php class. This can be used to determine if a restored lock is owned by the current owner. Currently it is not possible to determine if a restoredLock is owned by the given owner, except when releasing it.

We ran into a problem when we wanted to validate on the queue if we still had the lock on a specific key. We pass the $owner to the job, and can currently release the lock using restoreLock($key, $owner)->release(), but we have no way of validating whether the lock is still viable (without releasing it).

This is backwards-compatible because it only adds a function to the api, not changing any existing functions.

[taylorotwell]

Can't add to contracts on patch releases.