[PY-76681] Add id for PyClassNameCompletionContributor

[trinhanhngoc]

Hello @avokin,

I noticed that you recently made a experiment with extended completion. This change accidentally affected my plugin for PyCharm (https://plugins.jetbrains.com/plugin/13499-odoo). My plugin has a completion contributor which needs to take priority over other Python completion contributors to provide applicable completion items in injected Python expressions in XML. Because PyClassNameCompletionContributor now has order="first", adding a id for it will help other contributors to take priority over it if needed.

I have also created a similar PR for PyModuleNameCompletionContributor before.