feat: support start_time parameter on read_changes

[ewanharris]

Description

Adds the start_time parameter to the read_changes request

References

openfga/sdk-generator#455

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.95%. Comparing base (99f45c7) to head (e56a36f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   68.92%   68.95%   +0.03%     
==========================================
  Files         124      124              
  Lines        9853     9864      +11     
==========================================
+ Hits         6791     6802      +11     
  Misses       3062     3062              

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