[online_image] add option to show placeholder while downloading (#7083)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

esphome/components/online_image/__init__.py

@@ -27,6 +27,7 @@ CODEOWNERS = ["@guillempages"]
 MULTI_CONF = True
 
 CONF_ON_DOWNLOAD_FINISHED = "on_download_finished"
+CONF_PLACEHOLDER = "placeholder"
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -73,6 +74,7 @@ ONLINE_IMAGE_SCHEMA = cv.Schema(
         #
         cv.Required(CONF_URL): cv.url,
         cv.Required(CONF_FORMAT): cv.enum(IMAGE_FORMAT, upper=True),
+        cv.Optional(CONF_PLACEHOLDER): cv.use_id(Image_),
         cv.Optional(CONF_BUFFER_SIZE, default=2048): cv.int_range(256, 65536),
         cv.Optional(CONF_ON_DOWNLOAD_FINISHED): automation.validate_automation(
             {
@@ -152,6 +154,10 @@ async def to_code(config):
 
     cg.add(var.set_transparency(transparent))
 
+    if placeholder_id := config.get(CONF_PLACEHOLDER):
+        placeholder = await cg.get_variable(placeholder_id)
+        cg.add(var.set_placeholder(placeholder))
+
     for conf in config.get(CONF_ON_DOWNLOAD_FINISHED, []):
         trigger = cg.new_Pvariable(conf[CONF_TRIGGER_ID], var)
         await automation.build_automation(trigger, [], conf)