Unify express middleware APIs

[sidharthachatterjee]

As noted in #12924, there isn't an equivalent way to add middleware for serve like there is for develop (developMiddleware)

Adding a new API for this like we proposed earlier doesn't seem like a great idea and would expand the API surface unnecessarily. Instead we ought to unify these for the two in v3

[bradvogel]

Checking in on this - would you all be willing to take a PR to expose a new serveMiddleware configuration option? We're using a forked version of gatsby that exposes this option, just so we can get access to the express app and mount custom middleware.