mScene::mSkeletons not works in other formats but fbx

**Describe the bug**
I found that no data in aiScene::mSkeletons when I loaded from gltf file which contains a full skeleton. And I read the assimp source which verifed my find that assimp load data to the "mSkeletons" field only for .fbx. I want to know the reason why not support this field in other file formats. Thanks.

No source file or test is named. Start by comparing how the glTF and FBX importers populate aiScene::mSkeletons, then determine the intended cross-format skeleton representation; done should include a decided scope and coverage for supported non-FBX formats.

Written by the indexing model from the issue text.