Add spacing to surface distances calculations

[gasperpodobnik]

Is your feature request related to a problem? Please describe.
The current implementation of Hausdorff distance and other surface distance metrics only works for images with isotropic spacing.

Describe the solution you'd like
Consider adding a new parameter spacing to the metric call in order to pass it as sampling=spacing to both distance_transform_edt and distance_transform_cdt functions inside the get_surface_distance function (

MONAI/monai/metrics/utils.py

Line 175 in a8302ec

def get_surface_distance(seg_pred: np.ndarray, seg_gt: np.ndarray, distance_metric: str = "euclidean") -> np.ndarray:

). This will allow users to specify the voxel spacing during distance transform computation, which can be useful for handling different image resolutions.
The same is done in DeepMind's implementation (https://github.com/deepmind/surface-distance/blob/master/surface_distance/metrics.py#L254).