drivers: wifi: esp_at: escape SSID and PSK

According to [1], SSID and PSK need to be escaped:

  Escape character syntax is needed if SSID or password contains any
  special characters, such as , or " or \

Implement character escaping to fix connection attempt to WiFi APs
containing special characters as part of SSID, like "My_2,4GHz_AP".
Increase "connect command" buffer length to handle worst-case scenario of
all the SSID and PSK characters being special characters.

[1] https://docs.espressif.com/projects/esp-at/en/release-v2.4.0.0/esp32/AT_Command_Set/Wi-Fi_AT_Commands.html#id6



Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>