Model hub

[strakova]

Adds ModelHub, which manages local cache directory of LinPipe models and provides access to them.

Not yet included

• Some of helper code should have it's own classes, like sha256 computation and asset downloader. These will be later separate branches.
• Unpacking archives (unpack: true) and loading the model; get_model() currently returns nullptr, as the models are not implemented yet.
• Perhaps an offline mode to turn off checking for updated models.json.

[foxik]

LGTM!

As decided, we merge now and then go through the required changes later.