Mark internal configuration as internal boolean in ConfigDef.ConfigKey [DBZ-1528]

Migrated from [DBZ-1528](https://issues.redhat.com/browse/DBZ-1528)

Currently we don't mark internal configurations boolean flag here https://github.com/debezium/debezium/blob/master/debezium-core/src/main/java/io/debezium/config/Field.java#L380, as results, internal configurations such as {{internal.database.history.ddl.filter}} will be exposed as well. Thus, it will make sense to mark internal configurations as internal here: https://github.com/apache/kafka/blob/126230dad0ca9e4660ad2f1912935c41bce77ea9/clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java#L1082

Start with debezium-core/src/main/java/io/debezium/config/Field.java around line 380, then inspect the referenced ConfigDef.java location in Kafka. Trace how the internal boolean flag affects exposure of configurations such as internal.database.history.ddl.filter, and verify that internal configurations are no longer exposed.

Written by the indexing model from the issue text.