[bfdorch] Orchagent support hardware BFD (sonic-net#1883)

What I did
Implement bfdorch to program hardware BFD sessions via bfd SAI.
Add vs test for bfd sessions.

Why I did it
To support hardware BFD.

How I verified it
Configure hardware BFD sessions on virtual switches and physical devices and confirm the BFD session is programmed.

orchagent/Makefile.am

@@ -81,7 +81,8 @@ orchagent_SOURCES = \
             isolationgrouporch.cpp \
             muxorch.cpp \
             macsecorch.cpp \
-            lagid.cpp 
+            lagid.cpp \
+            bfdorch.cpp
 
 orchagent_SOURCES += flex_counter/flex_counter_manager.cpp flex_counter/flex_counter_stat_manager.cpp
 orchagent_SOURCES += debug_counter/debug_counter.cpp debug_counter/drop_counter.cpp