Rename FuturesApi GetFundingRate method to GetFundingRates

[hmG3]

It is possible to fetch funding rates for all symbols at once.
I am suggesting introducing this breaking change to be consistent with other exchange libraries.

[hmG3]

@JKorf Need your input here.
api/v1/contract/funding_rate without symbol parameter returns data property as an array of items. But providing symbol, data property represents a single object. I think you already handled that somewhere in other libraries. Could you point out which pattern you used?

Btw, currently GetSymbolsAsync() fails with a deserialization exception when the symbol parameter is provided.

[JKorf]

Hi, I don't really have a preference for this. I've used seperate methods for it and also resolving it within a single method. However in this case I would go with the two seperate method since you can just leavet the existing one as it is and add the second method without symbol parameter.

[hmG3]

Ok, I went with a separate methods