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.

Previously, the expiration caused QUIC connection finalization even if
there are application-terminated streams finishing sending data.  Such
finalization terminated these streams.

An easy way to trigger this is to request a large file from HTTP/3 over
a small MTU.  In this case keepalive timeout expiration may abruptly
terminate the request stream.

Previously, st->value was passed with NULL data pointer to header handlers.

While inserting a new entry into the dynamic table, first the entry is added,
and then older entries are evicted until table size is within capacity.  After
the first step, the number of entries may temporarily exceed the maximum
calculated from capacity by one entry, which previously caused table overflow.

The easiest way to trigger the issue is to keep adding entries with empty names
and values until first eviction.

The issue was introduced by 987bee4363d1.