[Fuel backend] Support downloading a model

[osrf-migration]

Original report (archived issue) by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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Model::Fetch() has a mostly empty implementation. It should download a model from the fuel server if that model doesn't already exist on disk. Saving to disk would require implementing LocalCache::SaveModel and making sure Model has access to the FuelClient instance of LocalCache.

In order to make it possible to test the library with a mock REST class, ModelPrivate must be given an instance of REST and LocalCache classes from FuelClient, which may be difficult. The instances might have to pass through ModelIter or LocalCache when so they can be given to created Model instances.

[osrf-migration]

Original comment by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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• Edited issue description

[chapulina]

ign fuel download --url https://fuel.ignitionrobotics.org/1.0/openrobotics/models/Ambulance