[Private] Design private data structure

[davdiding]

Find the best way to get the private account data from exchange API, need to take care of these things:

1. handle private key and signature process
2. whether to open a new class to handle private data

[davdiding]

OKX solution

1. Use SDK function to parse the key and secret and user headers to request the url.
2. Potential problem is that it may not accept post method.
3. May need to rethink the cls.create(**params) method of create the exchange instance.

[davdiding]

Binance solution