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/supervisor.go

@@ -0,0 +1,8 @@
+package plugin
+
+// Supervisor provides the interface for an object that controls the execution of a plugin.
+type Supervisor interface {
+	Start() error
+	Stop() error
+	Hooks() Hooks
+}