[$20] Add pureV5 flag on challenge

[ThomasKranitsas]

Add support for a flag named pureV5 inside the legacy property of the Challenge object that’s similar to the existing pureV5Task. Update all related endpoints and documentation.

Repo information:

Repo: https://github.com/topcoder-platform/challenge-api

Create a pull request against the develop branch

[rootelement]

Challenge https://www.topcoder.com/challenges/08807271-e038-4c1e-b132-d2d0a6ac5a5a has been created for this ticket.

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[ThomasKranitsas]

Challenge https://www.topcoder.com/challenges/08807271-e038-4c1e-b132-d2d0a6ac5a5a has been assigned to gauravseta.

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[Gauravseta]

@ThomasKranitsas PR #419

[ThomasKranitsas]

Payment task has been updated: https://www.topcoder.com/challenges/08807271-e038-4c1e-b132-d2d0a6ac5a5a
Payments Complete
Winner: gauravseta
Copilot: thomaskranitsas
Challenge 08807271-e038-4c1e-b132-d2d0a6ac5a5a has been paid and closed.

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