feat: add initial backend API

[DavianYang]

Description

fix #7

• added backend service
• added online compiler

Additional context

• [x] Since exec() function is not outputting the results, have to use stringIO and sdout

• further research for error handling and fast compiling need to be done

What is the purpose of this pull request?

• [ ] Bug fix
• [x] New feature
• [ ] Other

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[ydcjeff]

Please add __pycache__ to .gitignore.