Dev extension interface

[punith300i]

Refactored the code structure, enabled extension_interfaces and extensions to SAND code base.

• The extension interfaces are part of the extension_interface module. (assistant, search, export)
• The previous SAND plugins are now considered as extensions and are part of the extension module. (assistant, search, export, wikidata)
• drepr exports are part of export extension, added an output format parameter to export the relational data to different format specifications.
• Added Search interface template with required functions, which will be later extended to wiki data elastic search.

I have tested functionality of the extensions after refactoring the code, screenshots of the tests are given below.

Mtab Assistant Prediction API Test

Export Data API Test

Export Model API Test