Add getters to AllocationSizes

[DeltaEvo]

AllocationSizes can adjust the parameters given, it might be usefull to read them back to put a hard limit on the memory usage https://github.com/gfx-rs/wgpu/blob/afec4b73c8d5af2cbc9581ba163d143aa031daf8/wgpu-hal/src/dx12/suballocation.rs#L94-L95

Or even better, but I don't know if it's in the scope of gpu-allocator, allowing to configure a budget (upper limit) for the allocator, or something that could allow to get the size of the underlying allocation used prior to a allocate call (that would only return a size if the allocator needs to grow a block or allocate a new one and 0 if it can reuse a block)