[mlir][polynomial] split attributes into its own tablegen (#92613)

Out of tree we have other dialects that use the ring attribute, but we
get compilation errors when generating ops while pulling in all the
Polynomial tablegen ops (there's no `-dialect` flag in `mlir-tblgen` for
op generation like there is for attributes and types).

This PR simply moves the attributes into its own file, so it can be
included separately, and this also requires moving the dialect
declaration into its own file.