Add country code to the Location message (#53)

Some control parameters change depending upon the country of the
microgrid. While the country code can be derived from the
geo-coordinates, it is better to expose it via our APIs, so that the
behaviour of obtaining the codes is more consistent, and clients do not
need to worry about it.

closes #52

Author: tiyash-basu-frequenz

RELEASE_NOTES.md

@@ -11,7 +11,7 @@
 ## New Features
 
 * Added a new message named `Location`, representing the co-ordinates of a
-  geographical location.
+  geographical location and the corresponding country-code.
 
 * Added a new variant `COMPONENT_CATEGORY_RELAY` to the `ComponentCategory`
   enum.

proto/frequenz/api/common/location.proto

@@ -16,4 +16,7 @@ message Location {
 
   // Longitude ranges from -180 (West) to 180 (East)
   float longitude = 2;
+
+  // Country ISO 3166-1 Alpha 2
+  string country_code = 3;
 }