Add methods to create, get, publish and delete UI plugins

Overview

This PR adds several new methods to be able to programatically manipulate UI plugins:

In VCDClient:
- New method to add a new UI plugin, which picks the required metadata from a given ZIP file and performs the upload to VCD.
- New methods to retrieve all UI plugins, a specific UI plugin by its URN, or a specific plugin defined by its unique combination of vendor, plugin name and version.
In new type UIPlugin:
- New methods to update the given UI plugin. Only can update the Enabled, ProviderScoped and TenantScoped flags. The other properties are read-only and specified on plugin upload.
- New methods to publish and unpublish the given UI plugin to specific or all tenants.
- New method to delete the given UI plugin.

Tested on 10.4.2, 10,3.3, 10.3.0