doh2: process input headers

As of 5.2.0, the DoH(2) implementation ignores all headers (including some pseudoheaders).

At least some (pseudo)headers should be processed, e.g.:

  • content-type
  • :path (currently, any endpoint answers to DoH queries)

There could also be a mechanism for modules to request certain headers that would be passed along with a request.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information