Add FXMacroData return helpers

[roberttidball]

Summary

• adds download_fxmacrodata_prices and download_fxmacrodata_returns helpers to quantstats.utils
• fetches FXMacroData daily FX spot/reference values from /forex/{base}/{quote} and converts them into price or return Series
• supports explicit api_key plus FXMACRODATA_API_KEY / FXMD_API_KEY environment variables

Validation

• python -m pytest tests\test_utils.py -q -k FXMacroData
• python -m compileall -q quantstats\utils.py tests\test_utils.py
• git diff --check