debezium-connector-db2: z/OS integration

Currently Debezium db2-connector is not able to work with DB2 on z/OS databases.

We worked on implementation of z/OS support in scope of our project needs.

Below are the summary of changes we've done so far (based on debezium version 1.7.0-snapshot code) and tested on both DB2 LUW and z/OS.

We would like to discuss on those code changes more specifically.

To support working with two versions of DB2 three additional parameters were required for us (those params can be set as default to current implementation: LUW):
1) custom.db.type - version of DB2 ( property for DB2Connector to manage type of DB2. VALUES: ZOS, other value will be interpreted as LUW)
Added additional parameter (custom.db.type) into config of connector,
read this parameter in io.debezium.connector.db2.Db2Connection and use it for SQL forming  (two branches in code: to use SQLs for Z/OS or for LUW)
2) custom.cdc.program.schema - CDC schema for capture program (table IBMSNAP_REGISTER is stored in this schema in both versions of DB2)
Added additional parameter (custom.cdc.program.schema) into config of connector,
read this parameter in io.debezium.connector.db2.Db2Connection and use it for SQL forming.
3) custom.cdc.table.schema - CDC schema where table with CDC records is stored.
Added additional parameter (custom.cdc.table.schema) into config of connector,
read this parameter in io.debezium.connector.db2.Db2Connection and use it for SQL forming.