Support wow<->fiat calculations

[sanderfoobar]

fiat-api.cakewallet.com does not support WOW, so USD<->WOW calculations wont work.

Relevant file:

https://github.com/cake-tech/cake_wallet/blob/e6b1da376d03594b7dd3b7dbe62438a13c685f76/lib/core/fiat_conversion_service.dart#L7-L8

The API currently supports the following cryptocurrencies:

curl -s 'https://fiat-api.cakewallet.com/v1/rates?convert=USD' | jq '.data[] .symbol' | sort
"1INCH"
"AAVE"
"ADA"
"ALGO"
"AMP"
"APE"
"AR"
"ATOM"
"AVAX"
"AXS"
"BAT"
"BCH"
"BNB"
"BSV"
"BTC"
"BTT"
"BUSD"
"CAKE"
"CELO"
"CHZ"
"COMP"
"CRO"
"CRV"
"CVX"
"DAI"
"DASH"
"DCR"
"DOGE"
"DOT"
"EGLD"
"ENJ"
"EOS"
"ETC"
"ETH"
"FEI"
"FIL"
"FLOW"
"FTM"
"FTT"
"GALA"
"GMT"
"GNO"
"GRT"
"GT"
"HBAR"
"HNT"
"HOT"
"HT"
"ICP"
"KAVA"
"KCS"
"KDA"
"KLAY"
"KNC"
"KSM"
"LDO"
"LEO"
"LINK"
"LRC"
"LTC"
"MANA"
"MATIC"
"MINA"
"MIOTA"
"MKR"
"NEAR"
"NEO"
"NEXO"
"OKB"
"ONE"
"PAXG"
"QNT"
"QTUM"
"RUNE"
"SAND"
"SHIB"
"SOL"
"STX"
"THETA"
"TRX"
"TUSD"
"UNI"
"USDC"
"USDD"
"USDN"
"USDP"
"USDT"
"UST"
"VET"
"WAVES"
"WBTC"
"XDC"
"XEC"
"XEM"
"XLM"
"XMR"
"XRP"
"XTZ"
"ZEC"
"ZIL"

More info about Wownero: https://www.coingecko.com/en/coins/wownero