Add SSL_CTX_get_num_tickets.

CPython uses this function. Our implementation is slightly weird since
it leaks the clamping behavior, but probably not a big deal.

Update-Note: When this is merged into the internal repository, we can
simplify the CPython patches.

Change-Id: I291ddf852fb463bf02998fe04d0d0e8cb358dc55
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/53485
Commit-Queue: Bob Beck <bbe@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

ssl/ssl_lib.cc

diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index 0b3febf..bbcc3b1 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -3037,6 +3037,8 @@
   return 1;
 }
 
+size_t SSL_CTX_get_num_tickets(const SSL_CTX *ctx) { return ctx->num_tickets; }
+
 int SSL_set_tlsext_status_type(SSL *ssl, int type) {
   if (!ssl->config) {
     return 0;