include: Add voicecall multiparty hint method

include: Add voicecall multiparty hint method

On protocols that support creation of multiparty calls outside of
oFono's control, we need a way to detect multiparty call transitions.
This method adds a way for the driver to hint the core which calls
should be part of the multiparty call.

Right now this is relevant only to HFP.  Real modem hardware should not
be using this method.