add getUserObjFromUsername util function,

update all backends to use userObj,
add user backend manager wrapper which calls all linked backends dealing with user data,
list backend handlers for each realm

src/backends/pve.js

@@ -13,7 +13,8 @@ export default class PVE extends PVE_BACKEND {
 		this.#pveRoot = config.root;
 	}
 
-	async openSession (credentials) {
+	async openSession (user, password) {
+		const credentials = { username: `${user.id}@${user.realm}`, password };
 		const response = await global.pve.requestPVE("/access/ticket", "POST", null, credentials);
 		if (!(response.status === 200)) {
 			return response;