Generating a unitypackage using unity-builder

**Context**

Hello! Thank you for this wonderful GH action, supporting docs etc - great stuff. I am a library developer and publish a "unitypackage" that Unity developers can import into their project: https://github.com/mParticle/mparticle-unity-plugin

We would like to generate our unitypackage with GH actions.

**Suggested solution**

We have a bash script that we currently run locally, it looks something like this:

```
/Applications/Unity/Unity.app/Contents/MacOS/Unity \
-projectPath "${PWD}" \
-logFile \
-exportPackage Assets/foo.cs \
Assets/bar.cs
Example.unitypackage -nographics -batchmode -quit;
```

You can see a full example here: https://github.com/mParticle/mparticle-unity-plugin/blob/master/buildscript.sh

Seems like the critical parameter that the GH action would have to take is the list of assets to export.

**Considered alternatives**

We tried to hack our way around and run bash script in the docker image created by unity-builder but paused as it was getting too unsustainable.

**Additional details**

Supporting the development of unitypackage's/sdks would increase your product's TAM such that it's even more useful to library developers vs only app developers :-)