Implement a Protocol for Quantity-like

[mhvk]

Currently, we use

def get_value_and_unit(arg, default_unit=None):
    try:
        unit = arg.unit
    except AttributeError:
        return arg, default_unit
    else:
        return arg.value, unit

which, if a protocol existed, would be as simple as

def get_value_and_unit(arg, default_unit=None):
    return (arg.value, arg.unit) if isinstance(arg, QuantityAPI) else (arg, default_unit)

The question is what exactly the protocol should be. E.g., from https://github.com/astropy/quantity-2.0/pull/2/files#r1858839304

@runtime_checkable
class QuantityAPI(Protocol):
    value: Array
    unit: Unit

But perhaps this needs more discussion with other units package maintainers?