Add a mechanism for querying Debezium's state when embedded in a process [DBZ-535]

Migrated from [DBZ-535](https://issues.redhat.com/browse/DBZ-535)

I'm using Debezium embedded in my process in order to capture data from MySql. As part of the initialization process, I used schema_only snapshot mode and I would like to be able to know when Debezium finished running it's snapshot, since until that point in time I cannot capture data from MySql.
I would suggest some API for querying the embedded engine and for it to respond if it's initializing, finished snapshot or in error for some reason (for instance, since a replication slave with the same uuid is already found in MySql).

Currently the only way I can get this information is from reading the log, and that's not good enough for monitoring.

BTW, I really like debezium embedded in my process as it has helped us a lot in overcoming many challenges in CDC from MySql

Start by reviewing the embedded engine's public API and lifecycle behavior around schema_only snapshots. Trace how snapshot completion and initialization errors are currently reported through logs, then define what observable states a querying mechanism must expose and how completion can be verified.

Written by the indexing model from the issue text.