Add otherIdentifiers to Dandiset model

[candleindark]

This PR closes #358.

This PR define the otherIdentifiers field in the Dandiset model. It is a list of alternative Dandiset identifiers of a given Dandiset used identifying the Dandiset in other DANDI instances.

Note: The field provides the following extra JSON

• "readOnly" set to true as it should only be set by the server(s).
• "nskey" set to "dandi" as this property is particular to DANDI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.91%. Comparing base (cabfa3d) to head (498cdac).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
+ Coverage   97.89%   97.91%   +0.01%     
==========================================
  Files          18       18              
  Lines        2377     2398      +21     
==========================================
+ Hits         2327     2348      +21     
  Misses         50       50              

Flag	Coverage Δ
unittests	97.91% <100.00%> (+0.01%)	⬆️

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[Copilot]

Pull request overview

This PR adds an otherIdentifiers field to the Dandiset model to store alternative Dandiset identifiers used in other DANDI instances. The field is marked as read-only and server-managed.

Changes:

• Extracted DANDISET_IDENTIFIER_PATTERN constant for reusability
• Updated existing identifier field to use the new pattern constant
• Added otherIdentifiers field as a list of strings with pattern validation

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[Copilot]

@candleindark I've opened a new pull request, #362, to work on those changes. Once the pull request is ready, I'll request review from you.