tls: do not send IP literals in the SNI extension

[david-yu]

Part 6 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on #6, so the diff here is only this PR's commit.

Why

Callers that connect by address pass the address as tls_options::server_name. Both backends copied it into the SNI extension, which RFC 6066 §3 forbids for IP literals, and servers that validate the extension reject the ClientHello.

What

• tls: do not send IP literals in the SNI extension (bfaba39af) — both backends skip the extension for a literal, bracketed or not. Verification is unchanged: the literal is still matched against the certificate's IP SANs. The GnuTLS call's return value is now checked.

Testing

A plain socket reads the ClientHello the client sends: DNS names carry the extension, IPv4 and IPv6 literals do not.

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 ← this PR
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