Introduce generic video constant manager for plugins

Allow a plugin developer to get back constants values,
and reset constants deletions or additions.

shared/models/plugins/server/plugin-constant-manager.model.ts

@@ -0,0 +1,7 @@
+export interface ConstantManager <K extends string | number> {
+  addConstant: (key: K, label: string) => boolean
+  deleteConstant: (key: K) => boolean
+  getConstantValue: (key: K) => string
+  getConstants: () => Record<K, string>
+  resetConstants: () => void
+}