A new "store:..." prefix for the "ssl_certificate_key" directive allows
loading keys via the OSSL_STORE API.
The change is required to support hardware backed keys in OpenSSL 3.x using
the new "provider(7ossl)" modules, such as "pkcs11-provider". While the
engine API is present in 3.x, some operating systems (notably, RHEL10)
have already disabled it in their builds of OpenSSL.