Add TemplateRenderingProviders and update project references

[pournasserian]

• Updated FluentCMS.sln to replace project type GUIDs.
• Added new TemplateRenderingProviders projects.
• Updated GlobalSection in FluentCMS.sln for new projects.
• Added project reference to FluentCMS.Providers.TemplateRenderingProviders in FluentCMS.csproj.
• Registered ScribanTemplateRenderingProvider in Program.cs.
• Updated appsettings.json with new API key for ApiSettings.
• Refactored LayoutProcessor.cs to use ITemplateRenderingProvider.
• Updated FluentCMS.Web.UI.csproj to remove Scriban and add project reference.
• Updated ContentListViewPlugin.razor.cs to use ITemplateRenderingProvider.
• Updated FluentCMS.Web.Plugins.Contents.ContentViewer.csproj to add project reference and remove Scriban.
• Added new project files for TemplateRenderingProviders and TemplateRenderingProviders.Abstractions.
• Added ITemplateRenderingProvider interface.
• Added ScribanTemplateRenderingProvider class.
• Added ServiceExtensions for registering ScribanTemplateRenderingProvider.