http: Add callback to help logging of server certificates.

* common/http.c (http_session_s): Add field cert_log_cb.
(http_session_set_log_cb): New.
(http_verify_server_credentials): Call callback.
dirmngr: Print certificates on failed TLS verification.

* dirmngr/ks-engine-hkp.c (cert_log_cb): New.
(send_request): Set callback.
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We use the KSBA functions here because we have them anyway in Dirmngr.