Make middleware usage similar to that of the Connector SDK

### Feature description

This came out of a chat between @raulb and myself about https://github.com/ConduitIO/conduit-processor-sdk/pull/129.

As part of https://github.com/ConduitIO/conduit-connector-sdk/pull/198, we changed how the middleware is used in connectors. TL;DR the middleware is specified through a configuration struct, a connector doesn't have a Configure/Parameters method anymore, but returns a reference to the config object (and the SDK parses the configuration maps and sets the values into the referenced object).

To have a similar experience developing processors and make it easier for developers to get started with processors, we should change the processor SDK to do the same as the connector SDK does.