'Access-Control-Allow-Private-Network = true' header for http response

[chelo-kjml]

Google chrome browser (and I guess others too) are deprecating direct access to private network endpoints from public websites as part of the Private Network Access (PNA) specification.

Complete information here: https://developer.chrome.com/blog/private-network-access-preflight/

Please merge this change in order to add that header to responses. For now it is working on an ab test level, but is supposed to be released in a couple of google chrome versions.

[chelo-kjml]

@corydolphin Hey Cory, first of all thanks for this great library, it's a great addition to any flask application.

I just want you to know that I'm submitting this pull request about new preflight header google chrome is implementing, it would be great if you could release a new version of the library including it so we can use it from public python package index.

Thanks in advance.

[Sergio-Space]

I'm also facing this issue, this PR fixes it for me. @corydolphin is there any plan on merging?

[cristi8]

Any plans on releasing a new version with this soon? I guess a lot of people will have to have a local fork with this PR if this is not released soon.