Create a UI offering patterns that are contextual to the selected container, or the selected blocks container

When a template part (e.g. Header / Footer / Sidebar), or something like the Navigation block is selected, it may be helpful to offer ways to select patterns that suit the semantic purpose of the selected container.

Whatever solution we come up with for #28736 could be used here, but there is an additional use case to consider – what happens when the container is empty? In that case, perhaps the placeholder solution we come up with for #28735 could be utilised?

Expanding on this a little (we may need to split this in to a separate issue) – when a block inside a semantic container is selected, it may be helpful to expose both the "Single block transforms" and "Containers" UI patterns described in #28735 and #28736.

Example: Selecting a Site Title block in a Header template part might enable the user to discover a pattern that changes that block to a Site Title + Site Tagline combo, or: a more complex header pattern that includes search, navigation, logo, etc, which would replace the entire contents of the container.