[YUV AHB] Add toggle adding static sampler to external texture expansions

In Vulkan, static YCbCr samplers must be used to sample YCbCr AHBs. In
follow-up commits, support of JITing pipelines to use specific YCbCr
static samplers will be added. In order to have the backend support an
extra static sampler per ExternalTexture, an option is added in
ExternalTexture expansion to multiple bindings to also add a static
sampler.

 - Adds the Device::NeedsStaticSamplerForExternalTexture option and a
   toggle that can be used in Vulkan to force that option to be true.
 - Adds logic in BindGroupLayoutInternalBase to add a static sampler
   to the external texture expansion into multiple bindings.
 - BindGroups validate that each static sampler is compatible with its
   texture passed for "sampledTextureIndex". For static samplers
   expanded from external textures, no such texture is passed by the
   application since the "sampledTextureIndex" is for another expanded
   binding, so just skip that case.
 - Adds a placeholder sampler in DeviceBase, which is used as the static
   sampler before the pipeline JITing replaces it. This required updating
   the DestroyObjectTests to not use the default sampler descriptor as
   content-less object caching made the placeholder sampler be returned,
   that is referenced by the device and not destroyed until the device
   destruction.

Bug: 468988322
Change-Id: Ib1cacda0ed1f50eab09ce8f942b82921aa208e2b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/297176
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kyle Charbonneau <kylechar@google.com>