Allow hooking into command buffers on a backend-specific basis

[JMS55]

Is your feature request related to a problem? Please describe.
I'm trying to hook into FidelityFX Super Resolution 2, a Vulkan/DirectX12 library. This library handles all the dispatches, but requires that we pass it a VkCommandBuffer.

Describe the solution you'd like
Add a method to get a backend-specific VkCommandBuffer (for Vulkan, or the equivalent for DirectX12) from a &mut wgpu::CommandEncoder. I believe this is not exactly straightforward, as it seems like wgpu's CommandEncoder wraps many command buffers or something.

The user should also be able to set a debug label that will show up in debuggers like RenderDoc for subsequent commands issued on the command buffer.

[cwfitzgerald]

This is also obsoleted by #4067