Add probe_ops function for discovering available io_uring features

[jcsp]

This is code that I used in https://gitlab.com/putget-io/capra to auto-detect whether the kernel had the features I wanted -- sharing code in case this of interest to others.

It does break the abstraction a bit, as one needs to know which Rio functions use which IORING_OP codes in order to know what to probe for.

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