Adding content management sample.

[julianunity]

• Demonstrates weak referencing with WeakObjectReference<T> , WeakSceneReference and UntypedWeakReferenceId.
• Shows how to download, load, and unload assets and sub-scenes at runtime.
• Includes local and remote content workflows, with fallbacks and caching.

Provides step-by-step guides for:

• Loading meshes and materials dynamically.
• Building and serving content catalogs for remote distribution.
• Configuring scripting defines symbols for content delivery and diagnostics.
• Covers how to toggle between low- and high-fidelity sub-scenes in the editor to simulate runtime streaming behavior.

[paulb-unity]

some initial thoughts -
I would remove the ENABLE_CONTENT_DIAGNOSTICS & ENABLE_CONTENT_BUILD_DIAGNOSTICS script defines
there is nothing in the project when I first open it to indicate what I should do to see the sample run - maybe add a readme
the EntitySceneLoading build profile built fine but the player had errors and did not load anything