net: ipv6_addr equality and hash, ipv4_addr(socket_address) throws instead of terminating

[david-yu]

Part 8 of 10 of the IPv6 series, split out of #1 and rebased on current scylladb/seastar master. Based on #8, so the diff here is only this PR's 2 commits.

Why

Two gaps in the address types:

• ipv4_addr has operator== and std::hash; ipv6_addr had neither.
• ipv4_addr(const socket_address&) is implicit and has been noexcept since b72a0cbd0, but it delegates to a constructor that throws for an IPv6 address, so an IPv6 socket_address reaching an ipv4_addr parameter called std::terminate.

What

• net: add operator== and std::hash for ipv6_addr (ab7e301d0) — also drops the stale FIXME on network_stack::connect: the local address has followed the destination's family since 453d531b0.
• net: report an error instead of terminating on ipv4_addr(socket_address) (00e7787f0) — drops the noexcept; the native stack's make_bound_datagram_channel now throws for a non-AF_INET address like its unbound counterpart, instead of reaching an assert.

API

• ipv4_addr(const socket_address&) loses noexcept. Calls are unaffected; only code that requires the conversion to be noexcept would notice.
• std::hash<::sockaddr_in> and operator==(::sockaddr_in, ::sockaddr_in) stay (review on net, dns, tls: make Seastar work on IPv6-only and dual-stack hosts #1).
• No socket_address::wildcard(family, port) any more: inet_address(family) (669685c78) and ipv6_addr(port) already spell a family's any-address.

Testing

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 ← this PR
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