Support human pose annotation

**Is your feature request related to a problem? Please describe.**

I need to annotate human pose for pose estimation and alike problems.

**Describe the solution you'd like**

A key point with lines between connected key points to represent human body, e.g., head, body, left arm, right arm, left foot, right foot. It can be more complex to better model human body. Ideally it can support self defined human body structure, e.g., a set of key points with names, linking relationships within the key points.

**Describe alternatives you've considered**

just key point in Label Studio, there are 2 drawbacks
* lacking connecting lines between key points is not friendly for visualization
* it's hard to associate key points for 1 person, e.g., 2 people in the image and two heads, two bodies

**Additional context**

A sample annotation: [link](https://dataloop.ai/docs/create-annotation-point)