Allow adding categories in the patch command

[zhiltsov-max]

That would be good if the patch command also had the option. Perhaps a note could be written in the documentation in the merge chapter that the patch command can also merge datasets (incl. export of images).

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The script merges the annotation files and the images but unfortunately there is an error in the new annotation file. The first dataset contains 23000 images and the other dataset contains 3000 images. If the commands:

datum create
datum add ...
datum info

are executed, length should show 26000. In fact, 23000 is shown. I have previously transformed both datasets with

datum transform -t rename ...

Originally posted by @DP1701 in https://github.com/openvinotoolkit/datumaro/issues/542#issuecomment-965242577