* caddyhttp: Support sending HTTP 103 Early Hints
This adds support for early hints in the static_response handler.
* caddyhttp: Don't record 1xx responses
* push: Implement HTTP/2 server push (close#3551)
* push: Abstract header ops by embedding into new struct type
This will allow us to add more fields to customize headers in
push-specific ways in the future.
* push: Ensure Link resources are pushed before response is written
* Change header name from X-Caddy-Push to Caddy-Push