[DOP-21442] Add Excel API schema

[IlyasDevelopment]

Change Summary

• Added API schemas for creating and updating Transfers with file_format=excel.
• Included parameters for the Excel format:
  • include_header: bool = False (applicable for both reading and writing).
  • start_cell: str | None = None (applicable for both reading and writing).

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.