Implement EventsByTagQuery in PersistenceTestKitReadJournal

[ptrdom]

When attempting to implement runtime persistence plugin configuration for pekko-projection I ran into an issue where PersistenceTestKitReadJournal does not implement EventsByTagQuery, so it cannot be used to implement tests. Real persistence layers, like Cassandra, JDBC and Leveldb of course do implement EventsByTagQuery, but it would be quite useful if testkit was able to provide this behavior too. From a quick look it looks to me like an implementation similar to EventsByPersistenceIdStage should not be too complicated to do.

This is related to #1513 - need to have both EventSourcedBehavior and EventSourcedProvider be able to take configs at runtime to enable building typical applications using this functionality.

I could work on this feature if it were to be accepted.

[pjfanning]

Seems useful to me.