Expose a method to run an arbitrary function in our Angular context.

This is intended to allow callers to use the context to do things other
than serve requests. For example, a caller could use this during startup
to inspect something in the application and pre-generate certain responses,
or conditionally register middlewares in Express, or other such tasks.

This then becomes a building block for the existing middlewareWithAngular
function, that provides the same capability for the common case where
it's a middleware that needs the context.