Implements standardized plugin interfaces for UI contributions

[mdwigley]

Description

This PR introduces a set of plugin interfaces in the PlugHub.Shared project that allow plugins to contribute user interface elements to the host application in a structured and extensible way.

The following interfaces and supporting descriptors have been added:

• IPluginStyleInclusion – enables plugins to register StyleInclude resources (themes, icon sets, etc.).
• IPluginPages – provides a contract for contributing navigation pages (views, viewmodels, and metadata) to the application's main NavigationView.
• IPluginSettingsPages – defines contributions of plugin-specific configuration pages to the host’s Settings hub.
• Descriptor classes (PluginStyleIncludeDescriptor, PluginPageDescriptor, SettingsPageDescriptor) encapsulate metadata for each plugin contribution.

Related Issue

• Resolves: Implement plugin interfaces #67

Motivation and Context

Currently, plugin developers do not have a standardized way to contribute UI components to the host app. Each plugin has to integrate in an ad-hoc manner, creating tight coupling and maintenance challenges.

By introducing these interfaces, we provide:

• A clear, extensible contract for contributing styles, navigation, and settings pages.
• The ability for plugins to dynamically register UI components without requiring host modifications.
• A scalable foundation for plugin-driven UI extensibility moving forward.

How Has This Been Tested?

• NA

Screenshots (if appropriate):

• NA

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Asset change (adds or updates icons, templates, or other assets)
• Documentation change (adds or updates documentation)
• Plugin change (adds or updates a plugin)

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the core logic.
  • I have linked the project issue above.
• My change requires a change to the assets.
  • I have linked the asset issue above.
• My change requires a change to the documentation.
  • I have linked the documentation issue above.
• My change requires a change to a plugin.
  • I have linked the plugin issue above.