Add FXMacroData integration

[roberttidball]

Summary

• add an optional FXMacroData client for macro, FX, COT, commodity, session, and sentiment context
• export the client from the package root
• include a sample Backtrader strategy that fetches macro calendar context alongside Alpaca data

Validation

• python -m py_compile alpaca_backtrader_api\fxmacrodata.py sample\strategy_fxmacrodata_context.py
• git diff --check