Enable Early Hints 103 without upstream support

Describe the feature you'd like to add to nginx

As discussed in https://github.com/nginx/nginx/pull/326, https://github.com/nginx/nginx/issues/763, and the RFC at https://blog.nginx.org/blog/we-request-your-feedback-for-early-hints, we'd like to be able to write early hints into our conf.

Describe the problem this feature solves

Afaik popular frameworks like Flask are inherently unable to produce Early Hints due to WSGI limitations. Probably not the only group with those limitations, but certainly one of the larger ones.

Additional context

Our build / deploy pipeline already has our full asset manifest and is aware of any files that should be hinted early, and their exact names and locations, so adding them to an Ansible conf template and reloading nginx would be trivial, but unfortunately currently not possible. @arut if you feel comfortable adding the functionality you used for local testing in a future release, that'd be greatly appreciated.