Component-scoped configuration cascade for the accordion. Pass any
combination of feature factories in a parent component's viewProviders
array.
Unlike provideAccordionConfig (root-only), provideAccordionConfigAt
injects the parent injector's CNGX_ACCORDION_CONFIG value (resolves through
the priority chain - root provider, library defaults, or another
provideAccordionConfigAt further up) and merges the supplied features on
top. Descendant accordion instances see the merged config; sibling sub-trees
keep the inherited value untouched.