[C++] Add a Tensor logical value type with varying dimensions, implemented using ExtensionType

Support for tensor in Table, RecordBatch, etc. where each row is a tensor of a different shape (e.g images of different sizes), but of the same underlying type (e.g. int32). Implemented as an ExtensionType, so no need to change the format. 

I don't see needing each row being a tensor with a different number of dimensions, so if the implementation for that falls out easily of the use case with each row in the table having a tensor with the same number of dimensions, great. If it adds a lot of complexity, that case would be postponed.