Allow passing additional data from getStaticPaths to getStaticProps

[bugproof]

There are only discussions about this but I couldn't find actual feature request issue.

#11272
#10977
#11370

Imagine you'd like to create a new page for each .json file in a directory. This would allow you to e.g. iterate over .json files and import/read them in one go without doing it in getStaticProps.

In Gatsby, you have something like createPage function.

Consider this example:

https://github.com/benawad/test-nextjs-blog/blob/master/pages/blog/%5Bslug%5D.js#L20-L51