ddata: Add wildcard prefix subscriptions to Replicator

[pjfanning]

Port of akka/akka-core#31731. Adds support for subscribing to all keys matching a given prefix by using * at the end of the key id. New and existing matching keys both trigger notifications.

part of #2730

Key

• Added UnspecificKey — a generic Key[ReplicatedData] used to represent matched wildcard keys in notifications
• Added withId(newId) to Key abstract class, used internally to construct a concrete key instance for each matched wildcard key

Replicator

• Added wildcardSubscribers MultiMap (keyed by prefix, i.e. id with * stripped)
• Added private isWildcard / dropWildcard helpers; applied these consistently to existing durable/durableWildcards logic too
• receiveFlushChanges: wildcard subscribers are notified for changed keys; new wildcard subscriptions receive current values for all existing matching keys
• receiveUnsubscribe / receiveTerminated: clean up wildcardSubscribers correctly
• hasSubscriber(keyId) / getSubscribersIterator(keyId): new private methods combining direct and wildcard subscriber lookup

Usage

// Subscribe to all keys with the "counter-" prefix
replicator ! Subscribe(GCounterKey("counter-*"), self)

// Changed(key) notifications arrive for "counter-a", "counter-b", etc.
// key.id will reflect the actual matched key id