[TASK] Get extension key from Provider instead of Request

This allows the Provider to have a say in which extension context is used when rendering - while the AbstractFluxController is agnostic about this, ConfigurationProviders are perfectly okay being allowed to override extension context.
[DOC] Add missing variable type declaration
[TASK] Allow Exposed View to read extension key from setup if not given as argument

This can shorten calls to the method (given that some path collections already contain the extensionKey setup parameter) while still letting the extensionName parameter override any defined configuration. In many cases the extensionName parameter would have been read from the same setup that contains the extensionKey parameter.
[TASK] Make ExposedTemplateView read config same as ExposedStandaloneView
[TASK] Clean up a few default value returns in FluxService