[5] Cross platform automated testing (Add Windows and MacOS)

[nikhilsaraf]

Appveyor is a possible CI system we can use

Review: I used it for a couple small things. It's not as polished as other products, but it'll do the job. If I was choosing CI for windows and macOS I'd look at GitHub Actions. It supports them. I'd use that for Linux builds too, it works pretty well.

These projects are using it if you want an example of how to use: https://github.com/stellar/golistcmp https://github.com/stellar/go-xdr

(These two projects are setup different. The way golistcmp works will work with windows and mac builds, the way go-xdr is setup to build within a container won't.)