Add new 'user' namespace in Che plugin API

[evidolob]

Description

This namespace should contain methods which allow CRUD operations over Che user preferences. We already have this methods as Theia service, we need expose them to Che Plugin API.

This API is need for #13494

Proposed API:

  export namespace user {
       export function getUserPreferences(): Promise<Preferences>;
       export function getUserPreferences(filter: string | undefined): Promise<Preferences>;
       export function updateUserPreferences(update: Preferences): Promise<Preferences>;
       export function replaceUserPreferences(preferences: Preferences): Promise<Preferences>;
       export function deleteUserPreferences(): Promise<void>;
       export function deleteUserPreferences(list: string[] | undefined): Promise<void>;
  }

  export interface Preferences {
      [key: string]: string;
  }