Adjust snapshot SELECTs to exclude filtered columns

Debezium connectors allow to exclude specific table columns from change events, e.g. to exclude large BLOB columns or columns with sensitive data.

That's controlled via the column.blacklist option (soon to be renamed, see DBZ-2171). Currently those excluded columns are excluded by the Debezium engine (see usages of Filters#columnFilter()), but for the sake of efficiency, we should not select their value(s) to begin with during snapshotting. The default snapshot SELECT produced for each table (see RelationalSnapshotChangeEventSource#getSnapshotSelect()) should therefore be adjusted to only select the non-excluded columns.

If a specific SELECT statement is given (see the snapshot.select.statement.overrides option), this one should take precedence.