Added functions wifi.getcountry() and wifi.setcountry()

[dnc40085]

Adds feature requested in #2312.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/en/*.

This PR exposes the SDK functions (wifi_set_country() and wifi_get_country()) that were introduced with SDK ver. 2.2.0.

[devsaurus]

Works as expected. I can connect to my test AP with hidden SSID on channel 13 after setting

wifi.setcountry({policy = wifi.COUNTRY_MANUAL})

Thanks a lot @dnc40085 for the quick response!

[marcelstoer]

Arnim do you think we need to mention this somewhere explicitly (apart from the release notes) or can you merge? I plan to create the master drop on Monday evening.

[devsaurus]

Without further details from Espressif, #2335 is the best "documentation" approach IMO.
Sorry for yet-another-issue 😕