Add proxy support for futures and spot clients (async and sync)

[Graeme22]

Summary

For context see: #256
This PR adds support for HTTP proxies to use with spot and futures REST clients, both sync and async.
For now websockets are omitted as the websockets library does not support proxies. The motivation is to allow for privacy/unblocking or other security-related concerns.

Changes / Features

A single argument, proxy, has been added to the constructors for all REST clients. This proxy is used for all requests placed with those clients. Tests have also been added for all four variations: SpotClient, SpotAsyncClient, FuturesClient, and FuturesAsyncClient. The most relevant changes are included in the kraken/base_api/__init__.py file and in the new tests.

Closes: #256

[btschwertfeger]

Hey @Graeme22, after applying the suggested changes, I'm happy with the proposed change.

I see that the CI jobs didn't start, have you activated those on your feature branch? Locally they just ran through, but having those demonstrated here as well would be preferable.

[Graeme22]

Hey @Graeme22, after applying the suggested changes, I'm happy with the proposed change.

Great!

I see that the CI jobs didn't start, have you activated those on your feature branch? Locally they just ran through, but having those demonstrated here as well would be preferable.

I just activated them, looks like you need to approve the changes for them to run.

[btschwertfeger]

I just activated them, looks like you need to approve the changes for them to run.

I can't see anything to approve for the actions. I'll check that behavior using another account tomorrow.

[btschwertfeger]

Merging by hand now - with failing tests - since they're just failing due to wrong setup which is not that important for this change. (Some environment variables are not set in the author's repository.)