add EUF plugin framework.

plugin setting allows adding equality saturation within the E-graph propagation without involving externalizing theory solver dispatch. It makes equality saturation independent of SAT integration.
Add a special relation operator to support ad-hoc AC symbols.

src/ast/euf/CMakeLists.txt

@@ -1,8 +1,14 @@
 z3_add_component(euf
   SOURCES
+    euf_ac_plugin.cpp
+    euf_arith_plugin.cpp
+    euf_bv_plugin.cpp
+    euf_egraph.cpp
     euf_enode.cpp
     euf_etable.cpp
-    euf_egraph.cpp
+    euf_justification.cpp
+    euf_plugin.cpp
+    euf_specrel_plugin.cpp
   COMPONENT_DEPENDENCIES
     ast
     util