CQRS showcase endpoint

[marioarce]

Add a CQRS showcase endpoint that walks the Axlis feature set, ported from the reference HowToAccessSitecoreItemsAsync.

• MediatR query and handler injecting ISitecoreFacade
• Pivot 1 Disclaimer: read TextFields
• Pivot 2 Home: ImageField value, MultilistField.As, lazy Axes, TextField
• Pivot 3 Dictionary: ItemReferenceField.AsItem
• Pivot 4 Axes: Parent, Children, Grandparent, Siblings, GetChildren<Style>
• Pivot 5 Descendants: GetDescendants<Style> via Folder
• One rich-API call (WithResult) exposing Value, Metadata, Diagnostics
• Controller action exposing the query; pivot paths in config (// fixme)

Refs #1