[mlir][vector] Clean up VectorTransferOpInterface (llvm#72353)

- 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
were overridden without others, the op would be have been broken.

Author: matthias-springer