[mlir][Vector] Add vector contraction to outerproduct lowering

This revision adds the additional lowering and exposes the patterns at a finer granularity for better programmatic reuse. The unit test makes use of the finer grained pattern for simpler checks.

As the ContractionOpLowering is exposed programmatically, cleanup opportunities appear and static class methods are turned into free functions with static visibility.

Differential Revision: https://reviews.llvm.org/D80375