Add TLS 1.3 session resumption to OpenSSL backend

[sauwming]

Summary

Adds TLS session resumption to the OpenSSL secure-socket backend, controlled by a new opt-in param pj_ssl_sock_param.enable_session_reuse (default off).

Previously every outgoing TLS connection performed a full handshake and the server side actively disabled session tickets. TLS 1.3 delivers the resumption ticket via a NewSessionTicket message after the handshake, and each outgoing connection uses its own SSL_CTX, so captured sessions are held in a process-global, mutex-protected cache keyed by server name and re-injected on the next connection to the same host.

When enable_session_reuse is set:

• Client caches sessions per server_name (via SSL_CTX_sess_set_new_cb) and resumes them (SSL_set_session), avoiding a full handshake.
• Server issues session tickets (including TLS 1.3 tickets), which were previously always disabled.

Changes

• ssl_sock.h / config.h: new enable_session_reuse param, session_reused info field, and PJ_SSL_SOCK_OSSL_SESS_CACHE_SIZE (default 64).
• ssl_sock_ossl.c: session cache + helpers (correct SSL_SESSION ref-counting, LRU eviction, pj_atexit cleanup for ASan), new-session callback, session injection in ssl_create, and gating of server ticket disabling on the flag. Guarded to OpenSSL ≥ 1.1.0.
• ssl_sock_imp_common.c: populate info.session_reused via SSL_session_reused().
• pjlib-test/ssl_sock.c: session_reuse_test() opens a ticket-issuing server and makes two client connections, asserting the first is a full handshake and the second resumes.

Testing

• OpenSSL backend and test compile warning-free (-Wall) against OpenSSL 3.4.1.
• pjlib + pjlib-test build and link cleanly.
• The new session_reuse_test runs as part of pjlib-test ssl_sock in OpenSSL-enabled builds (e.g. CI).

Notes

• Opt-in; default behavior is unchanged.
• Empty server_name (e.g. connecting by IP) → resumption is skipped.
• Not wired into pjsip_tls_setting in this change; lives at the pjlib pj_ssl_sock level.

[Copilot]

Pull request overview

Adds opt-in TLS session resumption support to PJLIB’s OpenSSL secure-socket backend by caching client sessions per server_name and allowing the server side to issue TLS session tickets (including TLS 1.3), plus a pjlib-test that validates “full handshake then resumed handshake”.

Changes:

• Introduces pj_ssl_sock_param.enable_session_reuse (default off) and exposes pj_ssl_sock_info.session_reused.
• Implements a process-global, mutex-protected OpenSSL SSL_SESSION cache keyed by server_name, with MRU/LRU eviction and atexit cleanup.
• Adds an OpenSSL-only pjlib-test that creates a ticket-issuing TLS 1.3 server and asserts the 2nd client connection resumes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pjlib/src/pjlib-test/ssl_sock.c	Adds TLS 1.3 session resumption test coverage for the OpenSSL backend.
pjlib/src/pj/ssl_sock_ossl.c	Implements session caching + injection and gates server ticket disabling behind the new flag.
pjlib/src/pj/ssl_sock_imp_common.c	Populates info.session_reused for established OpenSSL connections.
pjlib/include/pj/ssl_sock.h	Adds new API fields for enabling session reuse and reporting reuse status.
pjlib/include/pj/config.h	Adds configurable OpenSSL session cache size macro.