odb: add exists_prefix method

[skshetry]

Requires for testtool in dvc-data for supporting short_oid as an argument. I forgot to add in dvc-objects, thought I did.
It's not very optimal, but since we only require for one call in testtool, that should be fine.
iterative/dvc-data@3e0362a

[codecov-commenter]

Codecov Report

Merging #88 (b733da2) into main (1a03409) will increase coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   65.50%   65.78%   +0.28%     
==========================================
  Files          39       39              
  Lines        2406     2426      +20     
  Branches      374      379       +5     
==========================================
+ Hits         1576     1596      +20     
  Misses        801      801              
  Partials       29       29              

Impacted Files	Coverage Δ
src/dvc_objects/db.py	100.00% <100.00%> (ø)
tests/test_odb.py	100.00% <100.00%> (ø)

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