RPC over GCP (Google IoT Core) support

This change adds support for RPC using Google IoT Core mechanisms

 * Requests should be sent to the device as [commands](https://cloud.google.com/iot/docs/how-tos/commands).
 * Responses are published as telemetry events (to a subfolder).

```
$ mos --port gcp://project/region/registry/device call Sys.GetInfo
...
```

See https://github.com/mongoose-os-libs/rpc-gcp/ for details

CL: RPC over GCP (Google IoT Core) support

PUBLISHED_FROM=49b3c46770ccda9071873ddaaededd4d350ca140

mos.yml

@@ -30,6 +30,7 @@ conds:
         - origin: https://github.com/mongoose-os-libs/gcp
         - origin: https://github.com/mongoose-os-libs/ota-shadow
         - origin: https://github.com/mongoose-os-libs/rpc-azure
+        - origin: https://github.com/mongoose-os-libs/rpc-gcp
         - origin: https://github.com/mongoose-os-libs/rpc-mqtt
         - origin: https://github.com/mongoose-os-libs/rpc-service-ota
         - origin: https://github.com/mongoose-os-libs/watson

mos_cc3220.yml

@@ -5,6 +5,7 @@ libs:
   - origin: https://github.com/mongoose-os-libs/i2c
   - origin: https://github.com/mongoose-os-libs/gcp
   - origin: https://github.com/mongoose-os-libs/rpc-azure
+  - origin: https://github.com/mongoose-os-libs/rpc-gcp
   - origin: https://github.com/mongoose-os-libs/rpc-loopback
   - origin: https://github.com/mongoose-os-libs/rpc-mqtt
   - origin: https://github.com/mongoose-os-libs/rpc-service-gpio

mos_esp32.yml

@@ -17,6 +17,7 @@ libs:
   - origin: https://github.com/mongoose-os-libs/pwm
   - origin: https://github.com/mongoose-os-libs/rpc-azure
   - origin: https://github.com/mongoose-os-libs/rpc-gatts
+  - origin: https://github.com/mongoose-os-libs/rpc-gcp
   - origin: https://github.com/mongoose-os-libs/rpc-loopback
   - origin: https://github.com/mongoose-os-libs/rpc-mqtt
   - origin: https://github.com/mongoose-os-libs/rpc-service-atca

mos_esp8266.yml

@@ -11,6 +11,7 @@ libs:
   - origin: https://github.com/mongoose-os-libs/ota-http-server
   - origin: https://github.com/mongoose-os-libs/pwm
   - origin: https://github.com/mongoose-os-libs/rpc-azure
+  - origin: https://github.com/mongoose-os-libs/rpc-gcp
   - origin: https://github.com/mongoose-os-libs/rpc-loopback
   - origin: https://github.com/mongoose-os-libs/rpc-mqtt
   - origin: https://github.com/mongoose-os-libs/rpc-service-atca