Support overwriting an existing file at the same location

[oruebel]

Motivation

Fix #117
Fix #137

• Support overwriting an existing file at the same location when in w mode
• Expose the mode as a property of ZarrIO

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.67%. Comparing base (eae8fb5) to head (17c6ea1).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/hdmf_zarr/backend.py	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #229      +/-   ##
==========================================
+ Coverage   85.64%   85.67%   +0.03%     
==========================================
  Files           5        5              
  Lines        1198     1208      +10     
  Branches      297      301       +4     
==========================================
+ Hits         1026     1035       +9     
  Misses        113      113              
- Partials       59       60       +1     

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[oruebel]

@mavaylon1 this one is ready to review