Add type annotations and structure for trading-related classes in types.pyi

[river-walras]

Description of code - what bug does this fix / what feature does this add?

This commit adds comprehensive type annotations and structure for all trading-related classes in types.pyi. The changes include:

1. Added type hints for all trading data structures including:

   • Trade
   • Ticker
   • OrderBook
   • Liquidation
   • Funding
   • Candle
   • Index
   • OpenInterest
   • Order
   • OrderInfo
   • Balance
   • L1Book
   • Transaction
   • Fill
   • Position

2. Each class now has:

   • Properly typed class attributes
   • Type-annotated constructor parameters
   • Return type hints for methods
   • Support for Optional types where appropriate
   • Proper typing for raw data fields

3. Added support for:

   • Decimal types for financial values
   • Optional fields with proper typing
   • Union types for flexible data structures
   • Generic type parameters where needed

This change improves code maintainability and enables better IDE support through static type checking.

• - Tested
• - Changelog updated
• - Tests run and pass
• - Flake8 run and all errors/warnings resolved
• - Contributors file updated (optional)

The changes are purely additive and don't modify any runtime behavior, only adding type information for better development experience and code quality.

[bmoscon]

@River-Shi did you mean to close this? Do you no longer want to merge this?