Flink CDC connector release contains dependency with incompatible licenses

Currently, Flink CDC connector releases both slim and fat jars. Apart from CDC itself, all of its dependencies are packaged into fat jars, including some with incompatible licenses:

• Db2 connector: `com.ibm.db2.jcc:db2jcc:db2jcc4` licensed with a non-FOSS license (International Program License Agreement).
• MySQL connector: `mysql:mysql-connector-java` licensed with GPLv2 license, which is incompatible with Apache 2.0.
• Oracle connector: `com.oracle.ojdbc` licensed with a non-FOSS license (Oracle Free Use Terms and Conditions).
• OceanBase connector: `mysql:mysql-connector-java` licensed with GPLv2 license, which is incompatible with Apache 2.0.

To fix this problem we may:

1. Exclude questionable dependencies from released jar;
2. Add docs to guide user download & place dependencies manually.