Record fiat price information for the transactions and include it in export of transactions

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Merchants use blink to accept bitcoin payments and denominate prices in fiat, conversion is done automatically to sats.
For bookkeeping purposes the app provides the capability to export transactions as CSV file.
Currently the data included does not yet provide enough information for proper bookkeeping.

Include the requested fiat amount and for matters of precision additionally the used exchange price for the display currency.

There is a timestamp and it is easy to assume that it could be enough to lookup price information on external sources. Unfortunately the usually available price information is not available for the time but in hindsight for a very broad range of time.