feat(rustls): Expose negotiated key exchange group in SslDigest

## Use Case

We are implementing NIST SP 1800-37 *Addressing Visibility Challenges with TLS 1.3* forensic logging in a gateway built on Pingora. We need to log the negotiated key exchange group per connection to monitor post-quantum cryptography migration progress (per NIST IR 8547).

## Problem

`SslDigest` currently exposes `cipher` and `version`, but not the negotiated key exchange group. In rustls 0.23, this information is available via `ServerConnection::negotiated_kx_group()`, but Pingora does not expose the `ServerConnection` post-handshake, and `TlsSettings` constructs the `ServerConfig` internally with no user hook point.

## Proposed Solution (minimal, non-breaking)

Add an optional `kx_group` field to `SslDigest`:

`
ust
pub struct SslDigest {
pub cipher: Cow<'static, str>,
pub version: Cow<'static, str>,
pub kx_group: Option>, // NEW
pub organization: Option,
pub serial_number: Option,
pub cert_digest: Vec,
pub extension: SslDigestExtension,
}
`

Populate it from `ServerConnection::negotiated_kx_group()` (rustls) or the equivalent OpenSSL/BoringSSL call in the respective TLS stream implementation after handshake completes.

## Alternative (more flexible)

Allow users to provide a pre-built `Arc` to `TlsSettings`:

`
ust
impl TlsSettings {
pub fn from_server_config(config: Arc) -> Self;
}
`

This would enable custom `KeyLog` implementations, custom `CryptoProvider`, and other advanced TLS configuration use cases.

## Context

- Pingora version: 0.8.1
- rustls version: 0.23.x
- The `kx_group` is metadata only (no security implication to exposing it in the digest)
- Useful for: compliance logging, post-quantum migration monitoring, observability dashboards
- Related standards: NIST SP 1800-37, NIST IR 8547, NIST SP 800-52 Rev 2

Start at SslDigest and trace the TLS stream implementations through handshake completion, including the rustls ServerConnection::negotiated_kx_group() entry point and the TlsSettings construction path. Compare the rustls and OpenSSL/BoringSSL paths, then verify that the negotiated group is available in the digest without changing existing fields or behavior.

Written by the indexing model from the issue text.