A webhook trigger when a task is marked as completed

**Is your feature request related to a problem? Please describe.**
There is currently no straightforward way to trigger an external workflow when a task is completed in Label Studio.

**Describe the solution you'd like**
Add a Task Completed button that triggers a webhook when a task is marked as completed.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
I found a [similar issue](https://github.com/HumanSignal/label-studio/issues/4831) opened in 2023, but since it has been a while and there does not seem to be recent progress, I’m opening a new issue to revisit this request.