[reduce] Add op/dialect interfaces for providing reductions

Add an interface for dialects and operations to provide reduction patterns. As a first step these should probably be just simple lists of reduction patterns, but at a later point could be more elaborate, like operations being able to offer multiple, fine-grained reductions based on the circumstances of that operation.

This might eventually be something to upstream into MLIR proper.

The issue names no files, entry points, or tests. Start by locating existing reduction patterns and the interfaces used by CIRCT dialects and operations, then determine how a simple pattern-list API could be scoped; done means the design is agreed and the interface is implemented with coverage for the relevant providers.

Written by the indexing model from the issue text.