Add support for time entities (#6399)

* Add time entities

* Add tests

* Add myself to datetime codeowners

* Fix publishing times with 0 values

* Log performing TimeCall

* Implement `on_time` trigger

* Rename var

* Fix initial value for time

* Add arg name for clarity

* Remove useless checks

esphome/components/mqtt/mqtt_time.h

@@ -0,0 +1,45 @@
+#pragma once
+
+#include "esphome/core/defines.h"
+
+#ifdef USE_MQTT
+#ifdef USE_DATETIME_TIME
+
+#include "esphome/components/datetime/time_entity.h"
+#include "mqtt_component.h"
+
+namespace esphome {
+namespace mqtt {
+
+class MQTTTimeComponent : public mqtt::MQTTComponent {
+ public:
+  /** Construct this MQTTTimeComponent instance with the provided friendly_name and time
+   *
+   * @param time The time entity.
+   */
+  explicit MQTTTimeComponent(datetime::TimeEntity *time);
+
+  // ========== INTERNAL METHODS ==========
+  // (In most use cases you won't need these)
+  /// Override setup.
+  void setup() override;
+  void dump_config() override;
+
+  void send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) override;
+
+  bool send_initial_state() override;
+
+  bool publish_state(uint8_t hour, uint8_t minute, uint8_t second);
+
+ protected:
+  std::string component_type() const override;
+  const EntityBase *get_entity() const override;
+
+  datetime::TimeEntity *time_;
+};
+
+}  // namespace mqtt
+}  // namespace esphome
+
+#endif  // USE_DATETIME_DATE
+#endif  // USE_MQTT