quic: add Conn.ConnectionState

Add a method that returns the tls.ConnectionState for a connection.
Generally useful, and also required to let HTTP/3 expose the
ConnectionState in Requests.

Change-Id: Iba725e0f40c68020fc6ee45d49f5c609a2b6b493
Reviewed-on: https://go-review.googlesource.com/c/net/+/647075
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

quic/conn.go

@@ -186,6 +186,11 @@ func (c *Conn) RemoteAddr() netip.AddrPort {
 	return c.peerAddr
 }
 
+// ConnectionState returns basic TLS details about the connection.
+func (c *Conn) ConnectionState() tls.ConnectionState {
+	return c.tls.ConnectionState()
+}
+
 // confirmHandshake is called when the handshake is confirmed.
 // https://www.rfc-editor.org/rfc/rfc9001#section-4.1.2
 func (c *Conn) confirmHandshake(now time.Time) {