d3d12: Add mechanism for D3D12 Adapter Selection
This add a simple mechanism to select which GPU adapter the d3d12 driver should be using. A new environment variable is introduced.
This represent a substring to search for in the GPU descrition, for example "NVIDIA" or "INTEL", or "NVIDIA GeForce RTX 3090", etc...
GPU are searched in order and the first one to include the substring becomes a match. If no match is found, we default to the first enumerated GPU.