A price_fn backed by a fixed symbol -> USD map, for tests and for runs where
the operator supplies prices out of band.
price_fn = Raxol.Payments.Prices.Static.price_fn(%{"ETH" => "1700", "POL" => "0.07"})
price_fn.("ETH") #=> Decimal.new("1700")
price_fn.("UNKNOWN") #=> nil
Summary
Functions
Build a (symbol -> Decimal.t() | nil) from a symbol -> price map.