Add missing `linalg.batch_vecmat` named op (#70218)

Linalg currently has these named ops:
* `matmul`
* `matvec`
* `vecmat`
* `batch_matmul`
* `batch_matvec`

But it does not have:
* `batch_vecmat`

This PRs adds that for consistency, and I have a short-term need for it
( https://github.com/openxla/iree/issues/15158 ), so not having this
would cause some contortion on my end.