feat: Querybook GitHub integration backend

[zhangvi7]

Context

• Backend changes for Github integration querybook (endpoints, gh client, db changes)
• Depends on feat: Add oauth flow for querybook github integration pinterest/querybook#1497

Changes

• Datasource endpoints:
  • Link datadoc to github
  • Commit/pushing datadoc
  • Retrieving all previous versions/commits of datadoc
  • Restoring a previous version of datadoc
• GitHub Client
• Serialize/Deserialize datadoc util (json)
• Logic:
  • restore data_doc() - to persist restore previous version of datadoc
  • get and create_github_link()
• DB:
  • Added new data model to store GitHub link - 1:1 mapping between datadoc and github {repo, branch, file_path}
• Unit Tests
• Updated requirements/extra.txt for pygithub

Test Plan

• pytest querybook/tests/test_lib/test_github_integration/test_github_client.py
• pytest querybook/tests/test_lib/test_github_integration/test_serializers.py
• Manual E2E testing next PR