cli: add du command to show disk usage

[ranuzz]

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Implements a version of du command. It covers most of the relevant options from the original du and the output is a bit customized keeping the dvc use case in mind.

 dvc du --help
usage: dvc du [-h] [-q | -v] [-H] [-d [<integer>]] [-c] [-a] [-s] [--rev [<commit>]] [--json] url [path]

Summarize disk usage of the set of FILEs, recursively for directories tracked by DVC and by Git.
Documentation: <https://man.dvc.org/du>

positional arguments:
  url                   Location of DVC repository
  path                  Path to directory within the repository to list outputs for

optional arguments:
  -h, --help            show this help message and exit
  -q, --quiet           Be quiet.
  -v, --verbose         Be verbose.
  -H, --human-readable  print sizes in human readable format (e.g., 1K 234M 2G)
  -d [<integer>], --max-depth [<integer>]
                        print the total for a directory (or file, with --all) only if it is N or fewer levels below the command line argument; --max-depth=0 is the same as --summarize
  -c, --total           produce a grand total
  -a, --all             write counts for all files, not just directories
  -s, --summarize       display only a total for each argument
  --rev [<commit>]      Git revision (e.g. SHA, branch, tag)
  --json, --show-json   Show output in JSON format.

Example:

dvc du https://github.com/iterative/dataset-registry use-cases/cats-dogs -H
59.72 G          /use-cases/cats-dogs/data/train/dogs
59.72 G          /use-cases/cats-dogs/data/train/cats
119.45 G         /use-cases/cats-dogs/data/train
23.89 G          /use-cases/cats-dogs/data/validation/dogs
23.89 G          /use-cases/cats-dogs/data/validation/cats
47.78 G          /use-cases/cats-dogs/data/validation
167.23 G         /use-cases/cats-dogs/data
167.23 G         /use-cases/cats-dogs