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/manifest.go

@@ -0,0 +1,70 @@
+package plugin
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v2"
+)
+
+type Manifest struct {
+	Id      string           `json:"id" yaml:"id"`
+	Backend *ManifestBackend `json:"backend,omitempty" yaml:"backend,omitempty"`
+}
+
+type ManifestBackend struct {
+	Executable string `json:"executable" yaml:"executable"`
+}
+
+// FindManifest will find and parse the manifest in a given directory.
+//
+// In all cases other than a does-not-exist error, path is set to the path of the manifest file that was
+// found.
+//
+// Manifests are JSON or YAML files named plugin.json, plugin.yaml, or plugin.yml.
+func FindManifest(dir string) (manifest *Manifest, path string, err error) {
+	for _, name := range []string{"plugin.yml", "plugin.yaml"} {
+		path = filepath.Join(dir, name)
+		f, ferr := os.Open(path)
+		if ferr != nil {
+			if !os.IsNotExist(ferr) {
+				err = ferr
+				return
+			}
+			continue
+		}
+		b, ioerr := ioutil.ReadAll(f)
+		f.Close()
+		if ioerr != nil {
+			err = ioerr
+			return
+		}
+		var parsed Manifest
+		err = yaml.Unmarshal(b, &parsed)
+		if err != nil {
+			return
+		}
+		manifest = &parsed
+		return
+	}
+
+	path = filepath.Join(dir, "plugin.json")
+	f, ferr := os.Open(path)
+	if ferr != nil {
+		if os.IsNotExist(ferr) {
+			path = ""
+		}
+		err = ferr
+		return
+	}
+	defer f.Close()
+	var parsed Manifest
+	err = json.NewDecoder(f).Decode(&parsed)
+	if err != nil {
+		return
+	}
+	manifest = &parsed
+	return
+}