hashfile.tree: implement update_tree

[skshetry]

I am still thinking of what the high-level API should be for building these upserted files.

We need a way to build the files, with possible expansion of directories/prefixes and ignores applied. Also, would be nice to have a batched build functionality.

But since, build() is already complicated, I'm still thinking of other options for now.
So, I have only created a low-level helper to update a tree.

Related to iterative/dvc#4657.

[codecov-commenter]

Codecov Report

Patch coverage: 18.18% and project coverage change: -0.15 ⚠️

Comparison is base (a2f9ed9) 57.82% compared to head (6c00c1c) 57.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #344      +/-   ##
==========================================
- Coverage   57.82%   57.67%   -0.15%     
==========================================
  Files          51       51              
  Lines        3395     3405      +10     
  Branches      595      596       +1     
==========================================
+ Hits         1963     1964       +1     
- Misses       1346     1355       +9     
  Partials       86       86              

Impacted Files	Coverage Δ
src/dvc_data/hashfile/tree.py	53.07% <18.18%> (-1.98%)	⬇️

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[skshetry]

Closing for now, will reopen later if I need it.