Add TensorExt dialect with rotate op

[j2kun]

This is intended to be a generic rotate op that operates on any tensor type, mirroring the fhe.rotate from HECO, but operating in conjunction with secret.generic.

Later, a lowering from secret to BGV would have to lower an op like

secret.generic ins(%0: secret.secret<tensor<64xi16>>) {
^bb0(%arg1: tensor<16xi16>):
  %1 = tensor_ext.rotate %arg1, %c7
  secret.yield %1
}

where the rotation amount is a plaintext value defined in the ambient scope.