BE | Create UserPuzzles#create & Update UserLoans#create Endpoints

[MelTravelz]

Changes:

• Created UserPuzzles#create Endpoint
  • Assuming we can receive the puzzle_image_url from FE after they upload it via Cloudinary
• Update UserLoans#create Endpoint to not allow a loan creation if a puzzle status is pending or not available
• In handling a merge conflict with another incoming branch, also updated sessions_request_spec tests

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This is related to #34

This closes #9

[] I linted my work. (RuboCop)

[] I've updated documentation & README. (if necessary)

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