curl.h: add CURLOPT_CA_CACHE_TIMEOUT option

Adds a new option to control the maximum time that a cached
certificate store may be retained for.

Currently only the OpenSSL backend implements support for
caching certificate stores.

Closes #9620

include/curl/curl.h

@@ -2157,6 +2157,9 @@ typedef enum {
   /* websockets options */
   CURLOPT(CURLOPT_WS_OPTIONS, CURLOPTTYPE_LONG, 320),
 
+  /* CA cache timeout */
+  CURLOPT(CURLOPT_CA_CACHE_TIMEOUT, CURLOPTTYPE_LONG, 321),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;