Add static URL field in content library's item file struct

[jitinkumar94]

Is your feature request related to a problem? Please describe.
There are content library api changes introduced recently in which all files in library items will each have a static download URL.

Describe the solution you'd like
I would like to extend govmomi vmodl2 content library bindings to include this static download URL field.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

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