Upgrade to mockery v2

Go modules are the future and mockery v2 adds support for new features like exporting private interfaces as publicly usable mocks. Very nice features which aren't available in the original mockery.

Support for v1 of mockery

Supports the latest v1 of mockery (1.2.3), currently setting up support for mockery v2, as that is the modern version, but this release is being set up for compatibility with the original mockery/for people who still have issues with go modules.