[mlir][Linalg] Refactor Linalg op initTensors support - NFC

Manually-defined named ops do not currently support `init_tensors` or return values and may never support them. Add extra interface to the StructuredOpInterface so that we can still write op-agnostic transformations based on StructuredOpInterface.

This is an NFC extension in preparation for tiling on tensors.

Differential Revision: https://reviews.llvm.org/D88481