Add optional factory delegate support to IPluginStyleInclusion

[mdwigley]

Description

This change extends PluginStyleIncludeDescriptor with an optional Func<IStyle> factory.

The bootstrapper has been updated to handle both:

• URI‑based StyleInclude resources (existing behavior, unchanged).
• Factory‑produced styles (new behavior).

Factory styles are deduplicated by type to prevent multiple injections of the same theme.

This enables scenarios like shipping FluentAvalonia 2 (FA2) as a plugin rather than hard‑coding it in the host application.

Related Issue

• Resolves Support Factory Lambda for Avalonia Style Injection #117

Motivation and Context

Previously, only XAML resources could be included via ResourceUri.
Some libraries (e.g. FA2) require programmatic initialization.
Adding a factory delegate allows plugins to contribute styles and themes that cannot be expressed as static XAML includes.
This keeps the host app lean and makes theming pluggable.

How Has This Been Tested?

• Verified with a mock plugin that injects a simple TextBlock style via factory (all text turned red as expected).
• Verified with a mock plugin that injects a FluentAvaloniaTheme via factory (theme applied successfully).
• Confirmed that existing URI‑based includes continue to load without change.

Screenshots (if appropriate):

N/A

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.