Add lowering for iree_attention custom op

[NatashaKnk]

This PR is a follow-up for this PR in IREE-Jax in an effort to introduce a custom StableHLO attention op which lowers to the linalg_ext version.

[jpienaar]

I think we could even land this first with local testing & then use that as part of testing in the other repo.

[NatashaKnk]

By local testing, do you mean more extensive testing than exists here? Where should this be added if so?

[benvanik]

(closing as stale)