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/pluginenv/options_test.go

@@ -0,0 +1,32 @@
+package pluginenv
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/platform/plugin"
+	"github.com/mattermost/platform/plugin/rpcplugin"
+)
+
+func TestDefaultSupervisorProvider(t *testing.T) {
+	_, err := DefaultSupervisorProvider(&plugin.BundleInfo{})
+	assert.Error(t, err)
+
+	_, err = DefaultSupervisorProvider(&plugin.BundleInfo{
+		Manifest: &plugin.Manifest{},
+	})
+	assert.Error(t, err)
+
+	supervisor, err := DefaultSupervisorProvider(&plugin.BundleInfo{
+		Manifest: &plugin.Manifest{
+			Backend: &plugin.ManifestBackend{
+				Executable: "foo",
+			},
+		},
+	})
+	require.NoError(t, err)
+	_, ok := supervisor.(*rpcplugin.Supervisor)
+	assert.True(t, ok)
+}