message: Add methods for new agent2agent approach in agent header.
diff: extern "C" {
  #endif
  +extern indy_error_t indy_prep_msg(indy_handle_t command_handle,
  + const char *const sender_vk,
  + const char *const recipient_vk,
  + const indy_u8_t *const msg_data,
  + indy_u32_t msg_len,
  +
  + void (cb*)(const indy_u8_t *const encrypted_msg
  + indy_u32_t encrypted_len)
  + );
  +
  +extern indy_error_t indy_prep_anonymous_msg(indy_handle_t command_handle,
  + const char *const recipient_vk,
  + const indy_u8_t *const msg_data,
  + indy_u32_t msg_len,
  +
  + void (cb*)(const indy_u8_t *const encrypted_msg
  + indy_u32_t encrypted_len)
  + );
  +
  +extern indy_error_t indy_parse_msg(indy_handle_t command_handle,
  + const char *const recipient_vk,
  + const indy_u8_t *const encrypted_msg,
  + indy_u32_t encrypted_len,
  +
  + void (cb*)(const char *const sender_vk,
  + const indy_u8_t *const msg_data,
  + indy_u32_t msg_len)
  + );
  +
  /// Establishes agent to agent connection.
  ///
  /// Information about sender Identity must be saved in the wallet with indy_create_and_store_my_did
  