[datetime] Add const char * overloads for string parsing to avoid heap allocation (#13363)

esphome/components/datetime/date_entity.cpp

@@ -106,9 +106,9 @@ DateCall &DateCall::set_date(uint16_t year, uint8_t month, uint8_t day) {
 
 DateCall &DateCall::set_date(ESPTime time) { return this->set_date(time.year, time.month, time.day_of_month); };
 
-DateCall &DateCall::set_date(const std::string &date) {
+DateCall &DateCall::set_date(const char *date, size_t len) {
   ESPTime val{};
-  if (!ESPTime::strptime(date, val)) {
+  if (!ESPTime::strptime(date, len, val)) {
     ESP_LOGE(TAG, "Could not convert the date string to an ESPTime object");
     return *this;
   }