Be able to get the SNI from the HTTP filter context

## What is the problem your feature solves, or the need it fulfills?

I would like to be able to get the SNI when in the `req_filter()` callback, so that I can do some business logic on it with the HTTP HOST header.

## Describe the solution you'd like
In the `self` or more likely the `session`, store the SNI (e.g. in the downstream session) so it can be retrieved. The SSL Digest is available in the `req_filter` and that would be a suitable place to put it. (`session.digest();`)

## Describe alternatives you've considered
I tried to dive into the stream and get to the `get_ssl()` function, but was unable to figure out how (or if it is possible) to downcast `stream` to the `SslStream`.

```
if let Some(stream) = session.stream() {
println!("session stream: {:?}", stream);
println!("Actual type of stream: {}", std::any::type_name_of_val(&stream));
// downcast it somehow
// then
if let Some(ssl) = ssl_stream.get_ssl() {
if let Some(sni) = ssl.servername(pingora::tls::ssl::NameType::HOST_NAME) {
```
but the 5-6 methods I tried for downcasting didn't work. It would also be a lot of insider baseball even if it did work and likely fragile.

## Additional context
From looking at the code, the SNI is not extracted anywhere. It is available in the TLS callback via a call to the underlying ssl instance but since there's no HTTP context there's no place to put it.

```
#[async_trait]
impl pingora::listeners::TlsAccept for DynamicCert {
async fn certificate_callback(&self, ssl: &mut pingora::tls::ssl::SslRef) {
// Inspect the SNI
if let Some(sni) = ssl.servername(pingora::tls::ssl::NameType::HOST_NAME) {
println!("SNI: {}", sni);
}
ext::ssl_use_certificate(ssl, &self.cert).unwrap();
ext::ssl_use_private_key(ssl, &self.key).unwrap();
}
}
```

It's possible I missed an `ext` field somewhere where the SNI could be stuffed.