Make param ctrl_spec of get_ctrl_system mandatory

[anurudhp]

Makes custom override code much cleaner as the user doesn't have to check for None.

For example, a custom single controlled version would look like:

def get_ctrl_system(self, ctrl_spec: 'CtrlSpec') -> Tuple['Bloq', 'AddControlledT']:
    if ctrl_spec != CtrlSpec(): # default fallback
        return super().get_ctrl_system(ctrl_spec=ctrl_spec)
    
    ... # custom single controlled version.

instead of

def get_ctrl_system(
    self, ctrl_spec: Optional['CtrlSpec'] = None
) -> Tuple['Bloq', 'AddControlledT']:
    if not (ctrl_spec is None or ctrl_spec == CtrlSpec()):
        return super().get_ctrl_system(ctrl_spec=ctrl_spec)

    ... # custom

This does not affect bloq users, who can still use bloq.controlled(). It only makes it simpler for bloq authors to provide custom versions.

[tanujkhattar]

LGTM from my side -- The call site doesn't change with it; so users should still be able to do bloq.controlled() which is important.

cc @mpharrigan to see if there are any concerns and approve.

[mpharrigan]

lgtm, @anurudhp pinged me about this offline before :)