Add function calling REST example (#443)

* Add function calling REST example

* Update function calling sample to have file content in bash script

* Delete tools.json

* Move tools definition into the region tag.

---------

Co-authored-by: Mark Daoust <markdaoust@google.com>

samples/rest/function_calling.sh

@@ -0,0 +1,62 @@
+set -eu
+
+echo "[START function_calling]"
+# [START function_calling]
+
+cat > tools.json << EOF
+{
+  "function_declarations": [
+    {
+      "name": "enable_lights",
+      "description": "Turn on the lighting system.",
+      "parameters": { "type": "object" }
+    },
+    {
+      "name": "set_light_color",
+      "description": "Set the light color. Lights must be enabled for this to work.",
+      "parameters": {
+        "type": "object",
+        "properties": {
+          "rgb_hex": {
+            "type": "string",
+            "description": "The light color as a 6-digit hex string, e.g. ff0000 for red."
+          }
+        },
+        "required": [
+          "rgb_hex"
+        ]
+      }
+    },
+    {
+      "name": "stop_lights",
+      "description": "Turn off the lighting system.",
+      "parameters": { "type": "object" }
+    }
+  ]
+} 
+EOF
+
+curl "https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-pro-latest:generateContent?key=$GOOGLE_API_KEY" \
+  -H 'Content-Type: application/json' \
+  -d @<(echo '
+  {
+    "system_instruction": {
+      "parts": {
+        "text": "You are a helpful lighting system bot. You can turn lights on and off, and you can set the color. Do not perform any other tasks."
+      }
+    },
+    "tools": ['$(source "$tools")'],
+
+    "tool_config": {
+      "function_calling_config": {"mode": "none"}
+    },
+
+    "contents": {
+      "role": "user",
+      "parts": {
+        "text": "What can you do?"
+      }
+    }
+  }
+') 2>/dev/null |sed -n '/"content"/,/"finishReason"/p'
+# [END function_calling]