[EPIC] - Make PluginPresets the only interface for Plugin creation

[uwe-mayer]

Description

Currently we have 2 similar entities:

• Plugins: a single deployed instance of a Greenhouse Plugin
• PluginPresets: A suite of Greenhouse Plugins with the same configuration deployed to a fleet of Clusters

This is confusing to the user and more difficult to maintain. Also clients (mainly UI) need to make distinctions on CREATE or UPDATE.

Objectives

• Make PluginPresets the only interface for creating Plugins
• Maintain Plugins only as technical resource with no client/user interaction

Acceptance Criteria

• Client interaction with Plugins is denied
• PluginPresets allow selection of single Cluster
• PluginPresets show Status of dependent Plugins
• Migrate all Plugins from the CCloud Organization into PluginPresets
• Decide on how to deal with `Plugins deployed to the central Cluster: AdminPlugins

[IvoGoman]

Closing the #829. We still need to be able to create Plugins for now and should probably keep the possibility open in the future as well. This does not interfere with the plan to support just the creation of new PluginPresets from the Greenhouse UI