[mlir][Linalg] Add a polymorphic linalg.copy operation

With the recent improvements to OpDSL it is cheap to reintroduce a linalg.copy operation.

This operation is needed in at least 2 cases:
  1. for copies that may want to change the elemental type (e.g. cast, truncate, quantize, etc)
  2. to specify new tensors that should bufferize to a copy operation. The linalg.generic form
     always folds away which is not always the right call.

Differential Revision: https://reviews.llvm.org/D121230