fix: Introspection get_sequences() NotImplementedError

[shantanutomarr]

• When running migrate_schemas command that applies a migration to change the primary key field of a model from AutoField to BigAutoField, we get the following error:
  NotImplementedError: subclasses of BaseDatabaseIntrospection may require a get_sequences() method

• Apparently this method is not implemented in the tenant schemas backend yet, but is expected by Django 4.2.

• This commit adds that method by referring to earlier pull requests on this issue:

  • Added get_sequences so command flush works #567
  • Added get_sequences method #704

• Added comprehensive test case that reproduces the bug and verifies the fix works

Closes #538

[goodtune]

Thank you @shantanutomarr for your excellent contribution! Your implementation of the get_sequences() method correctly fixes the Django 4.2+ compatibility issue from #538.

I've created PR #712 which adds a comprehensive test case that:
✅ Reproduces the exact bug from issue #538 by directly calling introspection.get_sequences()
✅ Fails on master branch (demonstrating the bug exists)
✅ Passes with your implementation (proving the fix works)
✅ Provides regression protection for future Django versions

Your implementation is solid and identical to PR #704. To consolidate efforts and ensure proper test coverage, I'm closing this PR in favor of PR #712 which includes both the fix AND comprehensive testing.

The solution will be available once PR #712 is merged. Thank you for helping modernize django-tenant-schemas for current Django versions!