Adding tests and doc updates around column masking and truncating

The masking and truncating functionality is supported by now for all the relational connectors, by virtue of moving them to the shared FW (MySQL shares the relevant code, too), see DBZ-773. We didn't consistently add docs and tests, though, while doing so. Partly, this has happened by now:

• Documentation has been added in DBZ-1954
• Tests for truncating for Postgres have been added in DBZ-1685

There are still some loose ends, though:

• Add missing tests for column masking/truncating for Db2, SQL Server (added in DBZ-1972) and Oracle
• Add missing tests for column masking for Postgres (added in DBZ-1962 and DBZ-1685)
• Remove the odd definitions like public static final Field MASK_COLUMN(int length) in multiple connector config classes; apart from the name indicating it'd be a constant, they are only ever used in tests and thus should be moved to the test using them, or maybe a common helper (looks to have been refactored in DBZ-1972)
• In the docs we say repeatedly "Fully-qualified names for columns are of the form databaseName.tableName.columnName, or databaseName.schemaName.tableName.columnName"; I don't think that's quite true, I doubt the four particle form would actually work; rather it should be three particles with "databaseName" or "schemaName" as the first one, depending on the connector
• Related to the previous item, we say "DB_NAME.TABLE_NAME or SCHEMA_NAME.TABLE_NAME, depending on the specific connector" in the docs of message.key.columns; instead only the specific form should be given in each connector's doc