[5/6] [nexus] add an indent level to external-api http entrypoints (#…

…6461)

We're going to put all this in a trait soon, and this file is so large that I'd
rather not ruin blame. With this intermediate commit, it will be possible to
add this revision to `.git-blame-ignore-revs`, thereby preserving blame.

The diff is very large but it's not scary, literally just adding an indent via
a module.

(I should have also done this with some of our other APIs -- lesson
learned!)