Add SNI-based certificate selection to TLS 1.3 server handshake

[pomelio]

Add SNI-based certificate selection to TLS 1.3 server handshake

PR Description

This change adds Server Name Indication (SNI) support to the TLS 1.3 server handshake.

The server now parses the server_name extension from ClientHello, stores the requested hostname in handshake state, and can select a certificate dynamically through a new Options.sni_auth callback. If the callback does not return a certificate, the existing auth field is still used as the default fallback.

• Added Options.sni_auth to support dynamic certificate selection based on SNI.
• Added Handshake.server_name state to retain the hostname parsed from ClientHello.
• Implemented parsing for the server_name TLS extension.
• Changed server certificate selection to use the resolved SNI-specific certificate instead of always using opt.auth.
• Deferred signature scheme validation until after certificate selection, so validation is performed against the actual certificate chosen for the handshake.
• Added tests covering:
  • existing ClientHello parsing behavior
  • SNI parsing and SNI-driven certificate selection

Before this change, the server always used a single fixed certificate from opt.auth, regardless of the hostname requested by the client. That made virtual hosting impossible.

This patch enables:

• serving multiple hostnames from the same listener
• choosing the correct certificate during handshake
• preserving backward compatibility for callers that only use auth

New server option:

sni_auth: ?SniAuth = null

Where SniAuth is:

pub const SniAuth = struct {
ctx: *anyopaque,
selectFn: *const fn (ctx: *anyopaque, server_name: ?[]const u8)
?*CertKeyPair,
};

Behavior:

• selectFn is called after parsing SNI from ClientHello
• if it returns a certificate, that certificate is used
• if it returns null, the handshake falls back to auth

This change is backward compatible:

• existing users of auth continue to work unchanged
• sni_auth is optional

I could not use full zig test output as the validation signal because the current repository does not compile cleanly against the local Zig standard library version for unrelated reasons. The modified file passes formatting checks with:

zig fmt --check src/handshake_server.zig

[ianic]

I could not use full zig test output as the validation signal because the current repository does not compile cleanly against the local Zig standard library version

Use Zig version from build.zig.zon or the latest one 0.16.0-dev.3061+9b1eaad13.
Run example/run_sanity_check_tests.sh locally to pass the tests from ci.