http: build the Host header from an address in authority form

[david-yu]

Part 10 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on #10, so the diff here is only this PR's commit.

Why

request::make takes the Host header as a string, so a caller with a socket_address formats it, and the obvious "{}:{}" gives fd00::5:8080 for IPv6: a different address, not an authority (RFC 3986 §3.2.2 wants [fd00::5]:8080).

What

• http: build the Host header from an address in authority form (02c5867b2) — http::internal::format_authority(), which brackets IPv6 and drops a zone index (an authority cannot carry one), and request::make overloads taking a socket_address. The string overloads are unchanged.

Testing

test_format_authority: 10.0.0.1:80 unchanged, [2001:db8::1]:8080 bracketed, fe80::1%2 loses its zone, and request::make puts exactly that in Host and the URL.

On the core-aws VM (Ubuntu 24.04, GCC 14, C++23, RelWithDebInfo, GnuTLS and OpenSSL in one tree), this PR's head builds and passes dns, ipv6, socket, rpc, httpd, httpd_openssl, tls, tls_openssl, network_interface, websocket and unix_domain with SEASTAR_TEST_REQUIRE_IPV6=1, so none of their IPv6 cases skipped.

Stack

1. tests, ci: fail instead of skip when IPv6 is expected #2 tests, ci: fail instead of skip when IPv6 is expected
2. dns: reach nameservers over IPv6 #3 dns: reach nameservers over IPv6
3. dns: literals for any family, c-ares error text, link-local nameservers #4 dns: literals for any family, c-ares error text, link-local nameservers
4. net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers) #5 net, http, rpc: dual-stack listeners (IPV6_V6ONLY, IPv4-mapped peers)
5. net: IPv6 zones and strict literal parsing #6 net: IPv6 zones and strict literal parsing
6. tls: do not send IP literals in the SNI extension #7 tls: do not send IP literals in the SNI extension
7. tls: verify server_name on OpenSSL, opt-in #8 tls: verify server_name on OpenSSL, opt-in
8. net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating #9 net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating
9. net: listen on an unspecified address binds any family #10 net: listen on an unspecified address binds any family
10. http: build the Host header from an address in authority form #11 http: build the Host header from an address in authority form ← this PR