Add WebGPU dependencies and refactor *_TO_BUILD CMake options. (#7905)

Part of https://github.com/google/iree/issues/7840

`IREE_HAL_DRIVERS_TO_BUILD` and `IREE_TARGET_BACKENDS_TO_BUILD` now use "default" instead of "all".

The new WebGPU compiler target is included in the list of all targets but is _not_ enabled by default, and the Metal+CUDA targets similarly disable themselves on incompatible platforms. Developers must explicitly opt in to building the WebGPU compiler target or the Metal/CUDA targets in unsupported configurations. I'm using `-DIREE_TARGET_BACKENDS_TO_BUILD=CUDA;Dylib-LLVM-AOT;WASM-LLVM-AOT;Vulkan-SPIRV;VMVX;WebGPU`, for example.

Thanks to being conditionally enabled, we can also use CMake's [FetchContent](https://cmake.org/cmake/help/latest/module/FetchContent.html) instead of git submodules for some dependencies, which will _hopefully_ keep the rest of IREE's CMake and git infrastructure simple for most users while only adding costs for those us of working on or using these optional components.