Cache resource list

[chapulina]

Fuel-tools caches downloaded models into ~/.ignition/fuel, which is very convenient for reusing models repeatedly. Downloading the models every time wouldn't be efficient.

The FuelClient::Models function supports retrieving metadata for all models in a server, but this information is never cached. This means that when using an interface like the one on gazebosim/gz-sim#263 to visualize a model list, we always need to hit the server, and that call takes a long time until it gets all models for all users. Ideally, we'd cache that information, and give the user a button that they can use to force an update from the server.