Implement certificate compression (#95)

Certificate compression is defined in RFC 8879:
https://datatracker.ietf.org/doc/html/rfc8879

This implementation is client-side only, for server certificates.

- Fixes #104.

conn.go

@@ -1057,6 +1057,10 @@ func (c *Conn) readHandshake() (interface{}, error) {
 		m = new(endOfEarlyDataMsg)
 	case typeKeyUpdate:
 		m = new(keyUpdateMsg)
+	// [UTLS SECTION BEGINS]
+	case typeCompressedCertificate:
+		m = new(compressedCertificateMsg)
+	// [UTLS SECTION ENDS]
 	default:
 		return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
 	}