Implement the missing async functions of WebGPUDriver

**Is your feature request related to a problem? Please describe.**
Implementations of most of the async functions in WebGPUDriver (e.g., WebGPUDriver::createVertexBufferAsyncR) have been skipped so far with only TODO placeholders. We should implement them to advance the WebGPU backend.

**Describe the solution you'd like**
Solution: implement these functions.

**Describe alternatives you've considered**
N/A

**OS and backend**
Is this feature OS/backend specific? If so, explain why.
This is specific to the WebGPU backend.