[wifi] Avoid .rodata RAM cost for ManualIP on ESP8266

On ESP8266, .rodata is mapped to DRAM (RAM), not flash. When
StructInitializer is used with all compile-time constant fields,
the compiler places the entire struct as a const blob in .rodata,
silently consuming ~20 bytes of RAM.

Switch to field-by-field assignment on ESP8266 so the IP address
values are encoded as immediate operands in flash instructions
instead of a .rodata blob. Other platforms continue to use the
aggregate initializer since their .rodata is flash-mapped.

tests/components/wifi/test.esp8266-ard.yaml

@@ -1,6 +1,12 @@
 wifi:
   min_auth_mode: WPA2
   post_connect_roaming: true
+  manual_ip:
+    static_ip: 192.168.1.100
+    gateway: 192.168.1.1
+    subnet: 255.255.255.0
+    dns1: 1.1.1.1
+    dns2: 8.8.8.8
 
 packages:
   - !include common.yaml