[FEATURE] Support Private Network Access

[jub0bs]

Is there an existing feature request for this?

• [X] I have searched the existing feature requests

Is your feature request related to a problem? Please describe.

Private Network Access (PNA) is an extension of the CORS protocol that allows servers to instruct browsers to relax new restrictions enforced by Chrome on requests coming from a less private network (e.g. a request issued from https://example.com to http://localhost:8080). gorilla/handlers currently lacks support for PNA, which may compel its users, should they need that feature, to switch to a different CORS middleware library.

Describe the solution that you would like.

Add at least one (and perhaps two) functional option(s) to let users configure Private Network Access for their CORS middleware.

Anything else?

• jub0bs - Fearless CORS: a design philosophy for CORS middleware libraries (and a Go implementation)