Add startup_delay to interval. (#5327)

esphome/components/interval/interval.h

@@ -8,8 +8,26 @@ namespace interval {
 
 class IntervalTrigger : public Trigger<>, public PollingComponent {
  public:
-  void update() override { this->trigger(); }
+  void update() override {
+    if (this->started_)
+      this->trigger();
+  }
+
+  void setup() override {
+    if (this->startup_delay_ == 0) {
+      this->started_ = true;
+    } else {
+      this->set_timeout(this->startup_delay_, [this] { this->started_ = true; });
+    }
+  }
+
+  void set_startup_delay(const uint32_t startup_delay) { this->startup_delay_ = startup_delay; }
+
   float get_setup_priority() const override { return setup_priority::DATA; }
+
+ protected:
+  uint32_t startup_delay_{0};
+  bool started_{false};
 };
 
 }  // namespace interval