Add build workflow

[sebastienros]

This creates the build workflow which is triggered on new PRs targeting master and when a commit is done on `master.

It also has a workflow for publishing the packages but it's not final. This can be tested freely as it's not publishing the packages it builds but still assigns then as artifacts to be able to look at the results. It's using the va.b.c tag to set the version, but this can be removed to reuse the current logic (defined in source).

[sebastienros]

Once merged, to setup, in Settings, Branches, Branch protection rules, add for master then check "Require status checks to pass before merging", select "build" (the name of the workflow in the yml). It may only appear after it's run at least once (a commit on master should trigger it automatically).

[sebastienros]

In terms of speed you should notice that the GH actions are started much quicker than appveyor. And should also run faster.

[sebastienros]

CI fails on appveyor because this PR is adding a global.json that enforces users have .NET 9.0 (I assume it's required to build this repos, right?)

[dadhi]

@sebastienros By users you mean contributors? If so then yes.
For the clients it multitargets to net472;netstandard2.0;netstandard2.1;net6.0;net8.0;net9.0