WIP plugins: add ability to register plugins

shared/models/plugins/plugin-package-json.model.ts

@@ -0,0 +1,15 @@
+export type PluginPackageJson = {
+  name: string
+  description: string
+  engine: { peertube: string },
+
+  homepage: string,
+  author: string,
+  bugs: string,
+  library: string,
+
+  staticDirs: { [ name: string ]: string }
+  css: string[]
+
+  clientScripts: { script: string, scopes: string[] }[]
+}