Add mgos_cron_get_next_invocation()

Calculate the next fire date of a cron job after the specified date

Closes cesanta/mos-libs#12
Fixes cesanta/mos-libs#8

CL: cron: Add mgos_cron_get_next_invocation()

PUBLISHED_FROM=c34804d354c2e3a779ffc917858714342676df66

include/mgos_cron.h

@@ -31,6 +31,7 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+#include <time.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -56,6 +57,11 @@ typedef void (*mgos_cron_callback_t)(void *user_data, mgos_cron_id_t id);
 mgos_cron_id_t mgos_cron_add(const char *expr, mgos_cron_callback_t cb,
                              void *user_data);
 
+/*
+ * Calculate the next fire date after the specified date.
+ */
+time_t mgos_cron_get_next_invocation(mgos_cron_id_t id, time_t date);
+
 /*
  * Returns whether the given string is a valid cron expression or not. In case
  * of an error, if `perr` is not NULL, `*perr` is set to an error message; it