[mlir][Linalg] Introduce linalg.pad_tensor op.

`linalg.pad_tensor` is an operation that pads the `source` tensor
with given `low` and `high` padding config.

Example 1:

```mlir
  %pad_value = ... : f32
  %1 = linalg.pad_tensor %0 low[1, 2] high[2, 3] {
  ^bb0(%arg0 : index, %arg1 : index):
    linalg.yield %pad_value : f32
  } : tensor<?x?xf32> to tensor<?x?xf32>
```

Example 2:
```mlir
  %pad_value = ... : f32
  %1 = linalg.pad_tensor %arg0 low[2, %arg1, 3, 3] high[3, 3, %arg1, 2] {
  ^bb0(%arg2: index, %arg3: index, %arg4: index, %arg5: index):
    linalg.yield %pad_value : f32
  } : tensor<1x2x2x?xf32> to tensor<6x?x?x?xf32>
```

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D93704