Add github action for automatic & manual builds

[Grimeh]

This adds a github action for building UnityDataTool:

• automatically on push to main, or
• manually when triggered (ie. workflow_dispatch)

Currently setup to do Windows x64 and macOS arm builds, as that likely covers the vast majority of users.
The automatic builds use release, the manual builds can choose between release or debug.

Inspired by @keijiro's work making an automated macOS action (at https://github.com/keijiro/UnityDataTools/blob/main/.github/workflows/build.yml).

[SkowronskiAndrew]

Thank you for submitting this PR @Grimeh, and keijiro who you mention had some working happening on this.

This looks like a very helpful way to start having some builds available as its rather a burden for users to have to compile themselves. I'll land this so we can start having some builds happening.

[Grimeh]

Awesome 🙂
Glad I could help.

Might be worth looking at making releases in the future since the action artifacts expire after some time; if a commit hasn't been pushed to main in a couple months there may not be any builds available for download.
I've never used Github's releases feature though so not sure what's involved there. Need to do some reading. First thought is that it may be possible/desired to create a release from the push action if building succeeds, but doing so for every push might be a tad overkill.

[SkowronskiAndrew]

Thanks @Grimeh, the action is working well.

That's a good point about releases - i didn't realize the action builds would age out, and its true that this really deserves a more visible "release" to download.

I think we are at a relatively stable state in terms of functionality right now, but have some ideas for new things landing in upcoming weeks. So I will look to establish an initial release marking the current state.

Initially i think I'll manually set up the release, but it seems that automating it through actions will be convenient if we do it on a more regular basis.