hook_civicrm_volunteer_projectDefaultSettings is not invoked in all contexts concerning default settings

Presently, the hook is invoked only when api.VolunteerUtil.getsupportingdata is called.

Our uses for default settings have expanded since this hook was created – namely, the defaults are referenced not just when settings defaults in forms but also during project creation (i.e., at the API level as well as the form level). It would be more appropriate for the hook to live in CRM_Volunteer_BAO_Project::composeDefaultSettingsArray().