Add change-item-price voting functionality

[PlayJeri]

Brief description

Added POST /change-item-price endpoint, along with new service methods for handling the price change voting. Added the new voting type and new item price fields for relevant DTOs.

Change list

• Added POST /change-item-price endpoint
• Added service methods to handle the new voting
• Added the new voting type to VotingType enum
• Added price field to CreateVotingDto
• Added tests for new logic
• Added handling cases for new voting type
• Added newItemPrice to StartVotingParams
• Changed the return type of cancelTransaction from false to null to match the correct IService return type
• Fixed existing test

[jkrizsan]

Looks Good!

[MikhailDeriabin]

Good work