Refactor connector

[AnastasiaShemyakinskaya]

• Extract interfacesLoader with Load function
• This interface will be implemented by PDFLoader, SampleLoader and JSONLoader structures
• Define persistent layer. Extract interface PersistenWriter with function Write.
• This interface will be implemented by FileWriter, ElasticSearchWriter and CSVWriter structures.
• Every structure implements logic for saving object in function Write
• Extract interface Service with function Save
• Interface will be implemented by StorageService. Save function will choose from existing PersistenWriter implementation and use the appropriate implementation.
• JSONLoader will contain Service interface as a structure fields
• Main function will choose appropriate Loader implementation depends on provided mod in arguments.

Approximate code structure:

pkg
├── loader
│   ├── interface.go
│   └── jsonloader.go
│   └── pdfloader.go
│   └── sampleloader.go
├── persistent
│   └── interface.go
│   └── filesystem.go
│   └── elasticsearch.go
│   └── csv.go
├── service
│   └── interface.go
│   └── storageservice.go