Feat/UI/introduce plugin overview

[uwe-mayer]

This PR introduces the following updates to the PluginDefinitions type:

• displayName: A human-readable label for the pluginDefinition.
• docMarkDownUrl: An URL to the markdown documentation file for this PluginDefinition
• icon: Icon specifies the icon to be used for this PluginDefinition in the Greenhouse UI. Icons can be either:
  • A string representing a juno icon in camel case from this list
  • A publicly accessable image reference to a .png file. Will be displayed 100x100px and grey scaled.

And some features to the plugin-admin view:

• A PluginDefinitions overview as Available Plugins Tab
• A PluginDefinition detail view
• A Plugin edit/create view, currently accessible from
  • the Configure Plugin button within a PluginDefinition detail view
  • clicking on a Plugin from the Enabled Plugins List within a PluginDefinition view

View README on instructions on how to spin up the ui with local mock resources representing all introduced changes.

The former plugindefinition-admin has been merged into the plugin-admin the "least intrusive way".

A couple of todos still need attention (not exclusive!):

• Merge two currently different stores into a single store
• Actually create a secret from a OptionValue with type secret
• Link the edit screen from the Plugin detail view
• Open Available Plugins tab with respective Plugin detail after successful Plugin create/update
• Smooth scroll on Panel open within PluginDefinition screens
• An option to delete a Plugin
• Use URLstate everywhere
• Hide empty entries in Plugin detail view.
• Watch Plugins instead of getting, so we get updates without browser reload
• Use a common interface for the Plugin object (see here)
• Style imported markdown in the PluginDefinition detail view.
• Check TODOs in code before merging
• Set docker-compose image tags to main before merging.

Unchecked boxes are moved to post MVP here:
#152

[uwe-mayer]

cc @edda @TilmanHaupt