feat(sftp): add deferrable mode to SFTPOperator

[venkata-pavani]

Summary

• Add deferrable parameter to SFTPOperator, allowing get and put transfers of a single file (or list of single-file pairs, when concurrency == 1) to be deferred to a trigger instead of blocking a worker slot while the transfer is in progress.
• Add SFTPOperatorTrigger (in airflow.providers.sftp.triggers.sftp) which performs the async file transfer via SFTPHookAsync and yields a TriggerEvent on success or failure.
• deferrable=True raises ValueError at execute-time for unsupported configurations:
  • operation == "delete"
  • concurrency > 1
  • no ssh_conn_id available (neither explicit ssh_conn_id nor an sftp_hook with one set)

This re-implements the design from #65480 (closed unmerged due to a rebase issue), scoped down per discussion on #65475.

Closes #65475

Test plan

• New unit tests added in providers/sftp/tests/unit/sftp/triggers/test_sftp.py (TestSFTPOperatorTrigger) covering serialization, get/put success, intermediate directory creation, and failure events.
• New unit tests added in providers/sftp/tests/unit/sftp/operators/test_sftp.py covering deferral behavior, the ValueError guard rails, and execute_complete.
• CI test suite (could not run pytest locally on this Windows checkout due to POSIX-only dependencies in airflow-core: fcntl, os.register_at_fork, and symlinked _shared packages).

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