Split build/test and publish stages into separate templates

[lbussell]

This is needed for https://github.com/dotnet/dotnet-docker-internal/issues/7413, because we need to call the publish stage separately from the build/test stages. I took the simplest approach, which is just making the build and publish stages their own templates, with their own inputs.

build-test-publish-repo.yml still contains some important default values, so the next step would probably be to push the default values down into the sub-templates. We can also pull the giant nested conditional out of publish.yml back up into build-test-publish-repo.yml, because that logic assumes that you aren't running the publish stage by itself.