Added register_signal method to sdbusplus::asio::dbus_interface
Added register_signal method to sdbusplus::asio::dbus_interface

Signals, just like other interface members, should be listed on
interface v-table. This change introduces ability to expose
information about signal defined by D-Bus interface.

Tested by calling function in following ways:
iface->register_signal<void>("Empty");                     // -
iface->register_signal<double,std::string>("TwoElems");    // ds
iface->register_signal<std::tuple<int, double>>("Struct"); // (id)
iface->register_signal<std::vector<int>>("Array");         // ai

Signed-off-by: Adrian Ambrożewicz <5d596128d78c438d49333a98311ba7a5bf287c3e@linux.intel.com>
Change-Id: I477ebdee0c4eb814e0c18987c96fe7d7c1eb90e6

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus