Create DocumentLoaderInterface

Problem/Motivation

We need helper document loader interfaces for abstracting things, like scraping web pages, loading pdfs, csv, word etc, loading Google Docs from the cloud.

Think about how Langchain is doing it: https://python.langchain.com/docs/integrations/document_loaders/

Proposed resolution

* Create a DocumentLoaderInterface (DocumentloaderInterface?)
* Create a plugin manager.
* Use similar OperationType scenario for them.
* Add events for them, so its possible to change parameters and log.
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