add download command to cache folder in advance

[haixuanTao]

To test out:

uv run robot_descriptions download iiwa14_description

[stephane-caron]

Thank you for proposing this feature 👍

nit: Maybe pull rather than download? The word "download" is already explicit 👌 I just feel "pull" would be consistent with git and other tools like docker or ollama. What do you think?

[coveralls]

Pull Request Test Coverage Report for Build 15780327442

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.99%

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Totals
Change from base Build 15760082148:	0.0%
Covered Lines:	1121
Relevant Lines:	1144

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💛 - Coveralls

[haixuanTao]

Made the change :)

thanks for the quick review :)

[stephane-caron]

LGTM, thank you for this contribution 👍