Add async callbacks to GLTFImportPluginContext

[eoineoineoin]

I had a use-case when implementing an import plugin and needed to call an async function. Attempting to do this from the synchronous callbacks would either trigger exceptions due to code which should only run on the main thread or would hang Unity.

This PR extends the GLTFImportContext to provide async versions of the callbacks; for backwards-compatibility, the base implementation calls the synchronous version, so any extant implementations of the GLTFImportPluginContext should not see any API changes due to this.

For my use-case, I just needed an async OnAfterImportNode(), but saw no reason that this shouldn't be supported for each callback.

[CLAassistant]

All committers have signed the CLA.

[hybridherbst]

Thanks for the PR! We'll take a look.

There is already helper class to help with running async code synchronously, would it be possible for your use case to instead run your async method synchronous inside the existing OnAfterImportNode?

The helpers are here: https://github.com/KhronosGroup/UnityGLTF/blob/4da23d4f010d1bbdfec0e024bb9f2758516ecf5f/Editor/Scripts/Internal/AsyncHelpers.cs#L16

[eoineoineoin]

Thanks for looking into this; wasn't aware of that AsyncHelpers at all - looks like it does the trick and works correctly; I think that's preferable to use, rather than making the plugins async. Think we can close this PR, but wonder if there's some way we could bring attention to that util, just for developer productivity?