[native][vulkan] Support samplers in ResourceTable

To store both samplers and images together in the same descriptor set
array, we use VK_DESCRIPTOR_TYPE_COMBINED_IMAGE_SAMPLER, and when adding
a texture view to the table, we set an "unused sampler", and similarly
when adding a sampler to the table, we set an "unused image".

Although this method wastes descriptor memory space, since each
descriptor stores both a sampler and an image, this keeps shader
emission simpler (one set of arrays per resource type at the same
binding, with simple indexing), and allows us to store all descriptors
in one descriptor set with
VK_DESCRIPTOR_BINDING_VARIABLE_DESCRIPTOR_COUNT_BIT enabled.

Also:

- Added two more e2e ResourceTableTests, one that adds a simple sampler
  to the resource table, and another that adds multiple samplers, making
  sure they sample a texture that's also added to the resource table
  correctly.

Bug: 473354063
Change-Id: I7fa8e89497fc12722c84f9136046ec6bc5dabba3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/294735
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>