Update wifi ssid / pw over MQTT

**Describe the problem you have/What new integration you would like**
Update wifi ssid / pw over MQTT, similar to what ap / captive_portal does, but then remotely via an MQTT publish command.

**Please describe your use case for this integration and alternatives you've tried:**
I have a good number of ESPHome devices deployed to different remote sites that communicate over MQTT via WAN to a centrally hosted MQTT broker. Remote sites (mostly student dorms) have a high wifi-density (many different accesspoint / ssid's) and the ability to remotely update ssid/pw would save on unnecessary site visits to fix connection issues.

**Additional context**
Related to https://github.com/esphome/feature-requests/issues/987

Review the existing ap/captive_portal Wi-Fi credential flow and the related feature request #987 first. Define the MQTT publish command and its credential-update behavior, then verify that remote sites can update SSID and password without a local visit.

Written by the indexing model from the issue text.