[SMART CONTRACT] Implement On-chain Dutch Auction

[EDOHWARES]

Description
A contract for token sales where the price starts high and decreases over time until a buyer is found.

Requirements and context

• Deterministic price decay function (Linear or Exponential).
• Immediate execution upon bid match.
• Batch sale support (Multiple items at decreasing prices).

Suggested execution

• Work in contracts/dutch_auction/.
• Implement buy(amount) with price relative to time.

Example commit message
feat: implement price-decaying dutch auction contract