core/service: support credential refreshing on reload

Closes #21099

src/basic/unit-def.c

@@ -223,6 +223,7 @@ static const char* const service_state_table[_SERVICE_STATE_MAX] = {
         [SERVICE_RUNNING]                    = "running",
         [SERVICE_EXITED]                     = "exited",
         [SERVICE_REFRESH_EXTENSIONS]         = "refresh-extensions",
+        [SERVICE_REFRESH_CREDENTIALS]        = "refresh-credentials",
         [SERVICE_RELOAD]                     = "reload",
         [SERVICE_RELOAD_SIGNAL]              = "reload-signal",
         [SERVICE_RELOAD_NOTIFY]              = "reload-notify",

src/basic/unit-def.h

@@ -132,6 +132,7 @@ typedef enum ServiceState {
         SERVICE_RUNNING,
         SERVICE_EXITED,                 /* Nothing is running anymore, but RemainAfterExit is true hence this is OK */
         SERVICE_REFRESH_EXTENSIONS,     /* Refreshing extensions for a reload request */
+        SERVICE_REFRESH_CREDENTIALS,    /* ditto, but for credentials */
         SERVICE_RELOAD,                 /* Reloading via ExecReload= */
         SERVICE_RELOAD_SIGNAL,          /* Reloading via SIGHUP requested */
         SERVICE_RELOAD_NOTIFY,          /* Waiting for READY=1 after RELOADING=1 notify */