[mlir] separable registration of operation interfaces

This is similar to attribute and type interfaces and mostly the same mechanism
(FallbackModel / ExternalModel, ODS generation). There are minor differences in
how the concept-based polymorphism is implemented for operations that are
accounted for by ODS backends, and this essentially adds a test and exposes the
API.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D104294