[mlir][ods] Add nested OpTrait

Allows for grouping OpTraits with list of OpTrait to make it easier to group OpTraits together without needing to use list concats (e.g., enable using `[Traits, ..., UsefulGroupOfTraits, Others, ...]` instead of `[Traits, ...] # UsefulGroupOfTraits # [Others, ...]`). Flatten in construction of Operation. This recurses here as the expectation is that these aren't expected to be deeply nested (most likely only 1 level of nesting).

Differential Revision: https://reviews.llvm.org/D106223