"Generate Lightmap UVs" does not expose options and does not override existing UV2

### Describe the bug 💬

Follow-up to #668. Unity's ModelImporter replaces UV2 when "Generate Lightmap UVs" is selected in the importer, UnityGLTF currently just uses the underlying `Unwrapping.GenerateSecondaryUVSet()` method, which seems to simply do nothing when there's existing UVs, so we probably need to delete the existing UVs first.

Additionally, it's probably a good idea to expose the parameters that go into that method, same as the FBX importer.

### Steps to reproduce 🔢

1. Import file that has UV2
2. Enable "Generate Lightmap UVs"
3. Note no new UVs are generated, in contrast to Unity's model importer

### Files to reproduce the issue ♻

See #668

### Editor Version 🎲

2021.3

### Render Pipeline and version

All

### UnityGLTF Version

2.17.1

### Operating System 👩‍💻

Windows, macOS, Linux

### When does this problem happen?

- [x] Editor Import
- [ ] Runtime Import
- [ ] Editor Export
- [ ] Runtime Export

### Additional Info 📜

_No response_

### Validations 🩹

- [x] I have [searched existing issues](https://github.com/KhronosGroup/UnityGLTF/issues): no issue already exist that reports the same problem.
- [x] I follow the [Code of Conduct](https://www.khronos.org/about/code-of-conduct)
- [x] I provided a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example), including files when necessary.

Start by locating the editor-import path that calls Unity's Unwrapping.GenerateSecondaryUVSet() and compare its available parameters with the Unity FBX importer. Reproduce the case from #668 with an existing UV2, then verify that generated lightmap UVs replace it and that the relevant generation options are exposed.

Written by the indexing model from the issue text.