Add support for associations
Add support for associations

General overview:
Monitor objects that implement org.openbmc.Associations.
React by creating objects implementing org.openbmc.Association.
Include org.freedesktop.DBus.ObjectManager support.

Details:
Implemented properties changed handler for changes to associations
property of org.openbmc.Associations objects.
Updated interfaces added/removed handlers to react to
org.openbmc.Associations.
Required an index to react to endpoint state changes.

For additional on associations information check:
https://github.com/openbmc/docs/blob/master/dbus-interfaces.md

bradbishop/phosphor-objmgr,bradbishop/phosphor-objmgr,bradbishop/phosphor-objmgr,openbmc/phosphor-objmgr,openbmc/phosphor-objmgr