Const added to members, chunked trailer support to HTTP/1, HTTP2 headers

HTTP2 headers

• pseudo headers are pushed into the front of the array list, and other than that, it will be treated the same as normal headers
• Trade off:
  • We know that push front to the array list is expensive. But, it should be used only few times, as you don't want to change pseudo headers a lot and there are at most 4 of them. More than that, we don't need to do the push front later when we need to send the headers into the wire.
  • The advantage of it is that we will have the mostly the same behavior as netty, which is used by Java SDK team already.
• add will push the pseudo header to the front of the list when needed (the last header is NOT pseudo header)

Chunked trailer

• Add chunked trailer support to HTTP/1

Const changes

• const added to aws_socket_options
• const added to aws_http_proxy_options
• const added to aws_tls_connection_options