Client:commands: Support for async commands

[Jomshir98]

Currently the commands framework only allows for synchronous handlers. It might be beneficial to expand this support to asynchronous code. Example usage might be waiting with commands completion until appearance action it triggers succeeded before reporting the result.

Parts of commands framework that should support async operations:

• Final command handler
• Argument parser
• [optional] Argument autocompletion (e.g. to allow getting values for autocomplete from the server on-demand)

The chat input should be frozen while the command/autocomplete is running with visual indication that there is progress if it takes longer than half a second. There should also be rather short timeout on the command (e.g. 10 seconds).