Takes an input layer, existing field and a new name for the field, and
outputs a new layer with the selected field renamed.
While this result could also be achieved with the Refactor Fields
algorithm, Refactor Fields isn't particularly model friendly. It
relies on a constant, fixed table structure, and can't adapt to
input tables with different field structures.
In constrast, this simple Rename Field algorithm adapts nicely for
model use, because it operates on a single field only and leaves
all the other fields untouched.