Add access to some fields of mbedtls_ssl_ticket

[mpg]

Since 3.0, all fields of mbedtls_ssl_ticket_key and mbedtls_ssl_ticket_context are now private. It turns out some applications where accessing them (originally reported in #5331):

lighttpd allows synchronization of session tickets
across multiple servers, and so writes into mbedtls_ssl_ticket_context to
manage mbedtls_ssl_ticket_context.
lighttpd mod_mbedtls_session_ticket_key_check()
https://git.lighttpd.net/lighttpd/lighttpd1.4/src/branch/master/src/mod_mbedtls.c#L335

It should be noted that ssl_ticket.c is only loosely coupled with the code TLS code, and applications who wish to modify how ticket protection is handled are free to write their own implementation of the callbacks passed to mbedtls_ssl_conf_session_tickets_cb(), and use their own data structures, to which they have full access.

However, in that instance it looks like the application only wants to slightly extend key management, so writing a full replacement to ssl_ticket.c would be a bit much for the task at hand, and it is desirable to allow applications to re-use most of ssl_ticket.c but just modifying the bits they need by accessing struct fields.

Adding direct read and write access to most fields of the structures involved is probably the simplest solution, with one exception: we don't want to expose the mbedtls_cipher_context_t member directly, as it's soon going to be replaced by a psa_key_id_t (as least when MBEDTLS_USE_PSA_CRYPTO is enabled), see #5203. So, instead of direct field access, we should provide setter and getter functions for the key material.

[gstrauss]

Submitted PR #5501 Add mbedtls_ssl_ticket_rotate() for ticket rotation

mbedtls_ssl_ticket_rotate() provides a mechanism for applications to control session ticket encryption key rotation without needing direct access to mbedtls_ssl_ticket_key struct members.