Add custom handling for argentinian blue dollar

[ebma]

• Creates a new directory custom that holds the implementation of the custom handler for the AMPE and ARS-USD (blue dollar) rate
• Adds an implementation for the binance API. Binance only rate limits per IP, and many public endpoints don't even require an API key so the implementation of the Binance client does not allow passing that. The endpoint for the price is documented here.
• To make sure that the FIAT:ARS-USD AssetSpecifier will receive a quotation from the custom handler (blue dollar) and not a fiat price from polygon, the supported custom assets are filtered from the list before passing it to the fiat and crypto handler in dia-batching-server/src/api/mod.rs

Closes #32.

[gianfra-t]

Looks good to me!

[TorstenStueber]

👍