Add a payload.get_header_length() method, remove header length definitions

src/libcharon/encoding/payloads/auth_payload.h

@@ -28,11 +28,6 @@ typedef struct auth_payload_t auth_payload_t;
 #include <encoding/payloads/payload.h>
 #include <sa/authenticators/authenticator.h>
 
-/**
- * Length of a auth payload without the auth data in bytes.
- */
-#define AUTH_PAYLOAD_HEADER_LENGTH 8
-
 /**
  * Class representing an IKEv2 AUTH payload.
  *