Enhance leverage example with detailed position management and collateral measurement

- Added functions to close existing BTC positions, set leverage with verification, and open new BTC positions.
- Improved output messages for better clarity on leverage settings and collateral usage.
- Implemented collateral measurement to compare used collateral at different leverage levels (5x and 10x).
- Refactored code for better organization and readability.

Author: alexey-kichin