fx-data-download.py: Dynamic list of assets

[kenorb]

Related:

• https://github.com/Leo4815162342/dukascopy-node
• https://github.com/Leo4815162342/dukascopy-node/blob/master/src/utils/instrument-meta-data/generated/instrument-meta-data.json
• curl -H 'referer: https://freeserv.dukascopy.com' 'https://freeserv.dukascopy.com/2.0/index.php?path=common%2Finstruments'