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/plugintest/api.go

@@ -0,0 +1,17 @@
+package plugintest
+
+import (
+	"github.com/stretchr/testify/mock"
+
+	"github.com/mattermost/platform/plugin"
+)
+
+type API struct {
+	mock.Mock
+}
+
+var _ plugin.API = (*API)(nil)
+
+func (m *API) LoadPluginConfiguration(dest interface{}) error {
+	return m.Called(dest).Error(0)
+}