Implement Gate.io Exchange

[Griffsano]

I propose to add Gate.io as new supported exchange.

The CSV export looks like this (UTF-16, tab separation):

No	Time	Trade type	Pair	Price	Amount	Total	Fee
1	2021-12-01 00:00:00	Buy	BTC/USDT	1.234	5.678	7.006652	0.1 BTC

I've already implemented the CSV read-in in book.py and currently working on the API for price_data.py: https://www.gate.io/docs/apiv4/en/index.html
Python SDK for the API: https://github.com/gateio/gateapi-python

However, it seems that the API does not support historic price data.
Instead, Gate.io offers downloads for historic quotations (CSVs for each month, which is not very practical): https://www.gate.io/developer/historical_quotes

Therefore, we may have to use the fallback solutions proposed in #15, #90.

Any ideas/suggestions how we could do this without the fallback solutions?

[scientes]

For price data: https://github.com/ccxt/ccxt
May work for gate.io #14 is an exchange agnostic Implementation of ccxt for cointaxman