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

.gitignore

@@ -16,6 +16,7 @@ web/static/js/libs*.js
 
 config/active.dat
 config/config.json
+/plugins
 
 # Enteprise imports file
 imports/imports.go
@@ -90,4 +91,4 @@ webapp/coverage
 tags
 .idea
 
-debug
+debug