[mlir][vector] Clean up VectorTransferOpInterface

- Better documentation.
- Rename interface methods: `source` -> `getSource`, `indices` -> `getIndices`, etc. to conform with MLIR naming conventions. A default implementation is not needed.
- Turn many interface methods into helper functions. Most of the previous interface methods were not meant to be overridden, and if some of the would have been overridden without other, the op would be have been broken.

Author: matthias-springer