Add a mechanism to specify a plugin as delayed load

This is adding support for delayed loading of ParaView plugins.

It adds the following:

• Concept of EnsurePlugin in vtkSMProxy and the XML declaration, that forces a plugin to be loaded when instancying VTK objects for a proxy
• Mechanism to automatically add EnsurePlugin when needed on XML declaration of proxies
• Logic to LoadDelayedPlugin in vtkPVPluginLoader
• Logic to parse the delayed_load attributes in plugin config file

See this discourse discussion for more info: https://discourse.paraview.org/t/paraview-plugin-delayed-load/13683/4

TODO:

• Add CMake logic to generate plugin config file
• Add CMake logic to generate plugin config file for delayed load feature
• Add logic to load a plugin config file from the UI
• Add logic to merge loaded plugin config file in the settings in that case
• Add doc
• Add testing

May be done later:

• Add logic to discover XML content of a plugin, available in the UI