Allow for custom_metadata for Tensor and SparseTensor IPC messages

The Schema IPC message allows custom_metadata. See here:

It would be great if one could specify similar custom_metadata for Tensor and SparseTensor messages. Currently if one wants to pass the "context/metadata" of a tensor, one has to find out-of-band solutions.

It would be nice, if this could be incorporated into the IPC flow. E.g. one might want to pass a timestamp associated with a tensor indicating when the data was obtained or geographical coordinates indicating the edges of a 2D tensor to locate the data geographically.