PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/api.go

@@ -0,0 +1,7 @@
+package plugin
+
+type API interface {
+	// LoadPluginConfiguration loads the plugin's configuration. dest should be a pointer to a
+	// struct that the configuration JSON can be unmarshalled to.
+	LoadPluginConfiguration(dest interface{}) error
+}