Add FXMacroData FX timeseries fetcher

[roberttidball]

Summary

• Add fetch_fxmacrodata_timeseries() for daily FX reference rates from FXMacroData.
• Cache results through pinkfish's existing timeseries cache path and return lower-case OHLCV columns plus adj_close.
• Export the helper from the package root and add a mocked unit test.

Validation

• python -m pytest tests\\test_fxmacrodata_fetch.py
• python -m py_compile pinkfish\\fetch.py pinkfish\\__init__.py tests\\test_fxmacrodata_fetch.py

[fja05680]

Thanks for the PR. Interesting addition for pinkfish. Merged.