cli: introduce basic dvc du

[efiop]

• By default shows du for every entry in path.
• -s|--summarize to show total.
• Sizes of directories themselves are considered 0.
• Humanized size by default.
• Sizes are as reported by fs/recorded in dvc files.

Example:

$ dvc du . data
285    data/__pycache__
826M   data/cats_dogs
11     data/.gitignore
826M   data/large
16.8M  data/tiny
18.4M  data/mnist
88.8M  data/small
1.73G  data

Note that we already have --size for dvc ls that just shows sizes.

Related https://github.com/iterative/studio/pull/6541
Fixes #4682

Docs coming in a separate PR.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (3c31234) 90.63% compared to head (6086fd5) 90.63%.

Files	Patch %	Lines
dvc/fs/dvc.py	74.07%	4 Missing and 3 partials ⚠️
dvc/repo/du.py	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10068      +/-   ##
==========================================
- Coverage   90.63%   90.63%   -0.01%     
==========================================
  Files         496      500       +4     
  Lines       37754    37847      +93     
  Branches     5492     5503      +11     
==========================================
+ Hits        34218    34302      +84     
- Misses       2900     2905       +5     
- Partials      636      640       +4     

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[skshetry]

@efiop, what is blocking this PR?

[efiop]

@skshetry Nothing, need to get back to it and add some tests.