[Python] Allow PyCapsule Interface in `pyarrow.scalar` constructor?

### Describe the enhancement requested

Should the `pyarrow.scalar` constructor check for the PyCapsule Interface? It would allow any input with `__arrow_c_array__`, additionally validating that the input is a length-1 array.

In this case it's less important for the _speed_ of sharing large data across multiple libraries than it is for the reliability of communicating Arrow data.

An existing workaround is to use `pa.array(other_scalar)[0]`, so it's not clear whether the `pa.scalar` constructor should or should not check for this.

cc @jorisvandenbossche

### Component(s)

Python