feat: add isolated margin adjustment endpoint

[Vinayak1337]

Fixes #89

What changed

• Added a signed POST /openApi/swap/v2/trade/positionMargin endpoint for adjusting isolated position margin.
• Exposed the endpoint through TradeService.adjustIsolatedMargin.
• Supported required symbol, amount, and adjustment type fields, plus optional positionSide, positionId, and recvWindow.
• Added focused tests for service dispatch, request shape, and optional-parameter omission.
• Marked the README feature checkbox as supported.
• Removed one unused import so lint passes.

Tests

• npm test -- trade.service.spec.ts
• npm run lint
• npm run build