Add user interface

[szczys]

This PR adds a blinking LED and button element for user interaction:

• One LED blinks at a rate that is configurable using BLINK_DELAY_MS in the Golioth Settings Service
• The two user buttons determine which LED is currently blinking
• When a button press changes which LED is blinking, the state is report to Golioth LightDB State service
• Issuing the get_wifi_info remote procedure call (RPC) will return information about current WiFi connection.