Consolidate Build and Release Pipelines

[pjbgf]

Consolidate all builds using a single platform - potentially GitHub Actions.

Context
Currently AppVeyor is being used for windows builds and CircleCI for linux.
Each platform tests for platform-compatibility, but also add the following activities:

• Windows pipeline: applies git semver and releases the nuget package.
• Linux: generate code coverage report and submit to codecov.