[Transformers] Transformation previews

[eminano]

Is your feature request related to a problem? Please describe.
Currently it's not possible to have a preview of the transformation rules before running the pgstream snapshot/replication commands, at which point it's harder to identify problems.

Describe the solution you'd like
Have a way of validating transformation rules against source data, as a preview, without writing it to the target.

Describe alternatives you've considered

• Dedicated pgstream preview command that with given transformation rules performs a preview of a selected number of rows from the source database and outputs them as csv or logging
• Option to provide previews at column level, where transformers can be validated individually with input data (no source connection required)