Add support for memory pressure events

[nical]

This is a low-priority item for Firefox. We'd like to have a way to signal to wgpu_core (and wgpu_hal) that we are running low on memory and that now is a good time to let go of optional allocations such as caches and do any kind of memory saving available.

For example the recently landed d3d12 shader cache (#7729) would be a good thing to hook up to the memory pressure event.

[ErichDonGubler]

Firefox downstream tracking: Bug 1954824 - MVP for responding to memory-pressure events in WebGPU