[serial_proxy] Add USB identity query for USB-bridged ports

Add SERIAL_PROXY_PORT_TYPE_USB_SERIAL, derived automatically when a
port's uart_id resolves to a usb_uart channel (never set by the user),
and a SerialProxyGetUsbInfoRequest/Response pair (IDs 153/154) that
reads VID/PID/bcdDevice and the manufacturer/product/serial strings
live from the descriptors the USB host stack caches, so a client can
identify the attached device before subscribing. Ports that are not
USB_SERIAL answer NOT_SUPPORTED; an unplugged device answers with
connected=false. ZigbeeProxyRequest moves to ID 155 (expected merge
order: set_mode, USB info, zigbee).

tests/components/serial_proxy/test-usb.esp32-s3-idf.yaml

@@ -0,0 +1,21 @@
+wifi:
+  ssid: MySSID
+  password: password1
+
+api:
+
+usb_host:
+
+# port_type is omitted deliberately: a port on a USB UART channel derives USB_SERIAL
+usb_uart:
+  - type: CDC_ACM
+    vid: 0x303A
+    pid: 0x831A
+    channels:
+      - id: usb_serial_channel
+        baud_rate: 460800
+
+serial_proxy:
+  - id: serial_proxy_usb
+    uart_id: usb_serial_channel
+    name: USB Serial Port

tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml

@@ -21,7 +21,6 @@ serial_proxy:
   - id: zigbee_usb_serial
     uart_id: zigbee_usb_channel
     name: Zigbee
-    port_type: TTL
 
 zigbee_proxy:
   serial_proxy_id: zigbee_usb_serial