[Python] Allow scalars when creating expression from compute kernels

We can create an expression (eg for a projection) using the compute kernels and passing expressions as arguments. But currently, all other arguments need to be expressions:

>>> pc.add(pc.field("a"), pc.field("b"))    # this works
<pyarrow.compute.Expression add(a, b)>
>>> pc.add(pc.field("a"), 1)   # this fails when passing scalar (same for pa.scalar(1))
...
TypeError: only other expressions allowed as arguments

You can still pass a scalar expression (pc.scalar(1), note pc. not pa.), but I think for scalars it would be a nice usability improvement if it's not needed to manually convert your python or pyarrow scalar to a scalar expression.