Better processors: processing any field

[maha-hajja]

Currently built-in processors can only manipulate fields Record.Key and Record.Payload.After. We should change processors so they can be configured to work on any of the openCDC fields, maybe even multiple fields at the same time (e.g. copy payload data into metadata).

[lovromazgon]

The new list of processors and the way they will address fields is described in the design document, will be tackled in #1290.