add `io_uring_wait_for_events` support

[dsh0416]

Description

This patch trying to expose the available_nr from internal APIs of liburing to make the API easier to be handled.

TODO

• [x] Impl of internal API changes
• [ ] Integrate with current uring backend
• [ ] Pass existing tests
• [ ] Check robustness on heavy load (especially if SQE is full)
• [ ] Prepare a version that may be merged to liburing library