[QQC-2962] Add a method to get label count for project

[mkozik-lb]

Adds a new method get_label_count to the Project class, which allows to effectively find how many labels are in the project. Our customers would do this before:

labels_from_labelbox = sum([1 for _ in target_project.labels()])

[vbrodsky]

Adds a new method get_label_count to the Project class, which allows to effectively find how many labels are in the project. Our customers would do this before:

labels_from_labelbox = sum([1 for _ in target_project.labels()])

I do not find the labels() method

[vbrodsky]

any error reporting? I.e. what happens if mutation blows up or returns an error?