this commit adds the ability to specify arbitrary HTTP headers
for either the Gateway or the API. simply set the desired headers
on the config:

    ipfs config --json API.HTTPHeaders.X-MyHdr '["meow :)"]'
    ipfs config --json Gateway.HTTPHeaders.X-MyHdr '["meow :)"]'

License: MIT
Signed-off-by: Juan Batiz-Benet <juan@benet.ai>