Collection assets removed on clone()

[cholden-ag]

Hello! First thanks for this wonderful library, it's been super helpful

I might be misunderstanding, but I think there is an issue with Collections losing their assets when you run .clone() because the attribute is not carried over when constructing a new instance. I saw some discussion about placing asset info in the Collection.extra_fields attribute (ref #293) but my understanding is that Collections now support having assets (e.g., there's a Collection.add_asset)

It looks to me like the asset attribute needs to be included in this call,
https://github.com/stac-utils/pystac/blob/main/pystac/collection.py#L556-L568
but was maybe omitted as the STAC spec evolved.

Example,

>>> import pystac
>>> coln = pystac.read_file("https://planetarycomputer.microsoft.com/api/stac/v1/collections/landsat-8-c2-l2")
# nice thumbnail!
>>> coln.assets
{'thumbnail': <Asset href=https://ai4edatasetspublicassets.blob.core.windows.net/assets/pc_thumbnails/landsat.png>}
# works
>>> coln.to_dict()['assets']
{'thumbnail': {'href': 'https://ai4edatasetspublicassets.blob.core.windows.net/assets/pc_thumbnails/landsat.png', 'type': 'image/png', 'title': 'Landsat 8 C2', 'roles': ['thumbnail']}}
# broken
>>> coln.clone().assets
{}

If this makes sense I'm happy to add a PR with a fix and tests

[duckontheweb]

@cholden-ag Thanks for the detailed bug report, and for tracking down the issues. I would happily review a PR with tests!

In Item.clone we handle transferring assets by calling item.add_asset(...) after the new Item is created, so I think we should follow the same pattern for Collection.clone.

Alternatively, I would support adding an assets argument to both Item.__init__ and Collection.__init__. I'm not sure why we don't include that as an optional argument already, but it seems like it would be nice to be able to create Items and Collections with assets in a single line.

[duckontheweb]

@cholden-ag Do you have bandwidth to put in a PR for this issue in the near future?