Store extension data alongside session ticket

gnutls_session_get_data* and gnutls_session_set_data* currently only deal with the bare session tickets, meaning that any metadata associated with it (e.g., max_early_data_size) needs to be manually preserved (see !1936 (closed)). As this is cumbersome and error-prone, maybe we could store the entire NewSessionTicket message instead. A tricky thing is that the format of the NST message is different in TLS 1.2 and TLS 1.3, maybe we could use TLS 1.3 version as it is more expressive.