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/hooks.go

@@ -0,0 +1,10 @@
+package plugin
+
+type Hooks interface {
+	// OnActivate is invoked when the plugin is activated.
+	OnActivate(API) error
+
+	// OnDeactivate is invoked when the plugin is deactivated. This is the plugin's last chance to
+	// use the API, and the plugin will be terminated shortly after this invocation.
+	OnDeactivate() error
+}