Make middleware usage similar to that of the Connector SDK

[hariso]

Feature description

This came out of a chat between @raulb and myself about #129.

As part of ConduitIO/conduit-connector-sdk#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.