CI: Replace AppVeyor with GitHub Actions

[vlsi]

GitHub Actions supports several PostgreSQL versions on Windows machines, so we could use them instead of AppVeyor: https://github.com/marketplace/actions/setup-postgresql-for-linux-macos-windows

AppVeyor is often slow, and it produced false positives too often.

It would be great if someone submitted a PR to replace AppVeyor with https://github.com/marketplace/actions/setup-postgresql-for-linux-macos-windows or something like that.

[ganesh47]

Can i work on this, seems like a good one for a first-time-contributor here, unless someone else is already at this ?

• Do you want this integrated with your existing workflow or can this be a new workflow, with the same triggers as defined in appveyor?
• Shall go over the contributor-guidelines, but believe, it should be good, if the CI job runs predictably with windows based tests......

[davecramer]

Can i work on this, seems like a good one for a first-time-contributor here, unless someone else is already at this ?
By all means please do

• Do you want this integrated with your existing workflow or can this be a new workflow, with the same triggers as defined in appveyor?
  No real requirements here
• Shall go over the contributor-guidelines, but believe, it should be good, if the CI job runs predictably with windows based tests......

Thanks!