[Lock] Add `LockKeyNormalizer`

[valtzu]

Q	A
Branch?	7.3
Bug fix?	no
New feature?	yes
Deprecations?	no
License	MIT

The Lock documentation mentions that the lock keys are serializable – however, it seems that's the case only if you're using native php serializer.

I'm proposing to add LockKeyNormalizer to make the example in the docs work the same when using Symfony Serializer too.