Add ESP32 Wifi Station Mode

[Autofix]

This PR configures the ESP32 in Station mode if the Station SSID is set. Otherwise it will default to configuring Access Point mode.

I've added sta_ssid and sta_pw in all locations where wifi_ssid and wifi_pw were used. These have also been renamed to ap_ssid and ap_ssid to make the different mode usage clear.

I've had to duplicate checks against the Station SSID in ssh-stamp-esp32/src/network/wifi.rs as we no longer pass the ap_config directly to fn wifi_up

[Autofix]

User flow is now:

1. Default to Access Point Mode.
2. The user can change to Station mode by setting SSH_STAMP_WIFI_STA_SSID. The access point must have DHCP for SSH Stamp to acquire a correct IP address.
3. When starting Station Mode it will attempt to connect for 10 seconds. If it doesn't connect it will clear the Station Mode SSID and restart back into Access Point Mode.
4. The user can return from Station Mode to Access Point Mode by clearing the Station Mode SSID. SSH_STAMP_WIFI_STA_SSID = ""

Both modes will report which Access Point and IP address the user needs to connect to.
If in Station Mode and it loses connection to the Access Point it will continually attempt to reconnect.

[brainstorm]

LGTM! Minor stuff to correct I reckon.

[brainstorm]

This should ideally go in settings.rs?

[Autofix]

I agree. I've changed this in PR #113