Added Google Docs Viewer Application

[jaivsh]

Added Google Docs Viewer project in Flutter Projects

I've added the Google Docs FLutter projects. It can open/load any google doc or pdf into the application, which the user can save locally or edit as per their requirement. Following plugins are used in this project:

flutter_pdfview shared_preferences http path_provider

Close #4019

Checklist:

• [x] I have mentioned the issue number in my Pull Request.
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have created a helpful and easy to understand README.md

[jaivsh]

@pranjay-poddar @MohitGupta121 @TusharKesarwani please review my PR