[FEAT] Make the IActivityDisplaySettingsProvider async to get settings from the API

[jeanbaptistedalle]

For now, the IActivityDisplaySettingsProvider can get settings from a GetSettings method. But it could be helpful to make the method async to be able to make some Http call to the API for example.

And then, we could develop later an API in Elsa.Core to get the Icon/Color, etc. (or in the describer API). So when an activity will be created in the API, nothing has to be change in Elsa.Studio to add an icon/color to this activity.