The directive limits the number of request headers accepted from clients.
While the total amount of headers is believed to be sufficiently limited
by the existing buffer size limits (client_header_buffer_size and
large_client_header_buffers), the additional limit on the number of headers
might be beneficial to better protect backend servers.

Requested by Maksim Yevmenkin.

Signed-off-by: Elijah Zupancic <e.zupancic@f5.com>
Origin: <https://freenginx.org/hg/nginx/rev/199dc0d6b05be814b5c811876c20af58cd361fea>

This allows to process a port subcomponent and save it in r->port
in a unified way, similar to r->headers_in.server.  For HTTP/1.x
request line in the absolute form, r->host_end now includes a port
subcomponent, which is also consistent with HTTP/2 and HTTP/3.

The $request_port variable contains the port passed by the client in the
request line (for HTTP/1.x) or ":authority" pseudo-header (for HTTP/2 and
HTTP/3).  If the request line contains no host, or ":authority" is missing,
then $request_port is taken from the "Host" header, similar to the $host
variable.

The $is_request_port variable contains ":" if $request_port is non-empty,
and is empty otherwise.

Missed in fcf4331a0.

RFC 9114, Section 4.3.1. specifies a restriction for :authority and Host
coexistence in an HTTP/3 request:

: If both fields are present, they MUST contain the same value.

Previously, this restriction was correctly enforced only for portless
values.  When Host contained a port, the request failed as if :authority
and Host were different, regardless of :authority presence.

This happens because the value of r->headers_in.server used for :authority
has port stripped.  The fix is to use r->host_start / r->host_end instead.

This might happen for Huffman encoded string literals as the result
of length expansion.  Notably, the maximum length of string literals
is already limited with the "large_client_header_buffers" directive,
so this was only possible with nonsensically large configured limits.

The change implements processing upstream early hints response in
ngx_http_proxy_module and ngx_http_grpc_module.  A new directive
"early_hints" enables sending early hints to the client.  By default,
sending early hints is disabled.

Example:

    map $http_sec_fetch_mode $early_hints {
        navigate $http2$http3;
    }

    early_hints $early_hints;

    proxy_pass http://example.com;

After fixing ngx_http_v3_encode_varlen_int() in 400eb1b628,
NGX_HTTP_V3_VARLEN_INT_LEN retained the old value of 4, which is
insufficient for the values over 1073741823 (1G - 1).

The NGX_HTTP_V3_VARLEN_INT_LEN macro is used in ngx_http_v3_uni.c to
format stream and frame types.  Old buffer size is enough for formatting
this data.  Also, the macro is used in ngx_http_v3_filter_module.c to
format output chunks and trailers.  Considering output_buffers and
proxy_buffer_size are below 1G in all realistic scenarios, the old buffer
size is enough here as well.