exchanges: Update Poloniex exchange API

[samuael]

PR Description

This pull request included a Poloniex exchange code implementation with REST and WebSocket capabilities. The Wrapper and Web-socket functions have all undergone testing. My code conforms to the GCT style standards used by other exchanges. golangci-lint was used to fix a few mistakes. For future use, endpoint methods that the GCT wrapper does not support are also implemented.

The PR included implementations for REST for trading and other endpoint methods, Web-socket streaming, and Trading through the web socket stream.

Fixes # (issue)

Type of change

Please delete options that are not relevant and add an x in [] as the item is complete.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How has this been tested

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration and also consider improving test coverage whilst working on a certain feature or package.

• [x] go test ./... -race
• [x] golangci-lint run
• [x] Test X

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation and regenerated documentation via the documentation tool
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally and on Github Actions/AppVeyor with my changes
• [ ] Any dependent changes have been merged and published in downstream modules