wgpu-hal: Expose CommandBuffer/CommandEncoder native API type

[Supreeeme]

Is your feature request related to a problem? Please describe.
wgpu does not currently handle all available options of all graphics apis - for example, when using Vulkan, doing a queue family transfer of an external image is not expressible in wgpu, so you have to fall back to manually creating a command pool and command buffer and using raw Vulkan to achieve it.

Describe the solution you'd like
Expose the native equivalents to CommandBuffer or CommandEncoder, so that the resulting command buffer can be submitted in the same queue submission as other wgpu CommandBuffers. For example, adding a function to CommandEncoder to support directly recording to the underlying VkCommandBuffer.

Describe alternatives you've considered
Manually creating command pool, buffer, and manually submitting to the queue. Depending on how many of these command buffers are needed, you could end up with a lot of queue submission overhead.

[cwfitzgerald]

Adding a raw_encoder or something wouldn't be to hard in wgpu-hal as a low-lift task.

For a more comprehensive writeup and plan of underlying api interop, look at #4067