stac copy --copy-assets does not copy collection assets

[constantinius]

Is your feature request related to a problem? Please describe.

stac copy --copy-assets only takes into account item assets, but collection assets are not copied.

Describe the solution you'd like

I would expect, that collection assets are treated the same way as item assets and are copied as well when the --copy-assets flag is used.

Describe alternatives you've considered

Maybe an additional flag can be used to handle the copying of collection assets, like --copy-collection-assets.

Additional context