**Motivation:**

In order to be able to receive the `SniCompletionEvent` sni mapping
needs to be used, however, the function
`QuicSslContextBuilder::buildForServerWithSni` doesn't allow setting
`ClientAuth` and alternativly the builder doesn't allow setting the SNI
mapping.

This means there is currently no way to use both SNI mapping and
support/enforce client certs with QUIC.

**Modification:**

Expose the `QuicSslContextBuilder::sni` so that its possible to create a
`QuicSslContext` with SNI mapping alongside other settings.

Also adds `QuicChannelConnectTest::testSniWithClientAuth` to validate

**Result:**

The following then works to allow both receiving the
`SniCompletionEvent` and require a client cert
```
// Build context with clientAuth and trustManager configured
QuicSslContext sniContext = QuicSslContextBuilder.forServer(key, null, cert)
    .applicationProtocols("alpn/1")
    .clientAuth(ClientAuth.REQUIRE)
    // ... 
    .build();

// Create SNI-enabled context using the builder's sni() method
QuicSslContext serverContext = QuicSslContextBuilder.forServer(key, null, cert)
    .clientAuth(ClientAuth.REQUIRE)
    .sni(hostname -> sniContext)
    .build();
```

Co-authored-by: Norman Maurer <norman_maurer@apple.com>