[3.12] gh-114572: Fix locking in cert_store_stats and get_ca_certs (GH-114573) (GH-115547)

gh-114572: Fix locking in cert_store_stats and get_ca_certs (GH-114573)

* gh-114572: Fix locking in cert_store_stats and get_ca_certs

cert_store_stats and get_ca_certs query the SSLContext's X509_STORE with
X509_STORE_get0_objects, but reading the result requires a lock. See
https://github.com/openssl/openssl/pull/23224 for details.

Instead, use X509_STORE_get1_objects, newly added in that PR.
X509_STORE_get1_objects does not exist in current OpenSSLs, but we can
polyfill it with X509_STORE_lock and X509_STORE_unlock.

* Work around const-correctness problem

* Add missing X509_STORE_get1_objects failure check

* Add blurb
(cherry picked from commit bce693111bff906ccf9281c22371331aaff766ab)

Co-authored-by: David Benjamin <davidben@google.com>