Open set labels support

**Is your feature request related to a problem? Please describe.**
I have a long-tail class/label set (1000+ classes, most of them rare), in an open set setup (I don't know all classes/labels in advance). In Labelstudio, I don't see how an operator can enter a new class name or find an existing class efficiently.

**Describe the solution you'd like**
Setup: computer vision.
I want the operator to select a polygon / bounding box and then start typing the label name into a textbox. Ideally, as the operator types, an interactive dropbox suggests known labels (a list filtered out from the known 1000+ names). If the operator clicks Enter instead of picking from the list, a new class should automatically be added to the list, and the object should be labeled with that class.

**Describe alternatives you've considered**
Didn't find any.

**Additional context**
I came up with a mock of what I have in mind: