Middleware

[lovromazgon]

Description

Introduces processor middleware and implements two middlewares:

• ProcessorWithSchemaDecode is the equivalent of DestinationWithSchemaExtraction
• ProcessorWithSchemaEncode is the equivalent of SourceWithSchemaExtraction

Fixes ConduitIO/conduit#1707.

Quick checks:

• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[raulb]

I personally like having the flexibility of separate middlewares so this could be easily extended by users (when the needs comes). Only left some nits. Great work @lovromazgon

[hariso]

AFAIC, cleared for takeoff.:)