feat: allow partially failing batch inserts

[hperl]

When batch-inserting multiple identities, conflicts or validation errors of a subset of identities in the batch still allow the rest of the identities to be inserted. The returned JSON contains the error details that lead to the failure.

Related issue(s)

Docs PR: ory/docs#1864

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 88.48921% with 32 lines in your changes missing coverage. Please review.

Project coverage is 78.64%. Comparing base (3215792) to head (efa05f6).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
persistence/sql/batch/create.go	81.25%	7 Missing and 8 partials ⚠️
persistence/sql/identity/persister_identity.go	84.00%	6 Missing and 6 partials ⚠️
identity/manager.go	96.00%	1 Missing and 1 partial ⚠️
persistence/sql/batch/test_persister.go	96.87%	1 Missing and 1 partial ⚠️
identity/handler.go	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4083      +/-   ##
==========================================
- Coverage   79.00%   78.64%   -0.36%     
==========================================
  Files         377      377              
  Lines       27492    26963     -529     
==========================================
- Hits        21720    21205     -515     
+ Misses       4165     4149      -16     
- Partials     1607     1609       +2     

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All fixed :)

[zepatrik]

Looks great now, just some small nits.