[mlir][linalg] Replace "string" iterator_types attr with enums in LinalgInterface.

[RFC: EnumAttr for iterator types in Linalg](https://discourse.llvm.org/t/rfc-enumattr-for-iterator-types-in-linalg/64535)

This affect touches and probably breaks most of the code that creates `linalg.generic`. A fix would be to replace calls to `getParallelIteratorTypeName/getReductionIteratorTypeName` with `mlir::utils::IteratorType::parallel/reduction` and types from `StringRef` to `mlir::utils::IteratorType`.

Due to limitations of tablegen, shared C++ definition of IteratorType enum lives in StructuredOpsUtils.td, but each dialect should have it's own EnumAttr wrapper. To avoid conflict, all enums in a dialect are put into a separate file with a separate tablegen rule.

Test dialect td files are refactored a bit.

Printed format of `linalg.generic` temporarily remains unchanged to avoid breaking code and tests in the same change.

Differential Revision: https://reviews.llvm.org/D137658