Adding count_tokens for rest (#444)

* Adding count_tokens for rest

* Update to have same prompt as python example

* tests now working

Author: shilpakancharla

samples/rest/count_tokens.sh

@@ -0,0 +1,32 @@
+set -eu 
+
+echo "[START tokens_text_only]"
+# [START tokens_text_only]
+curl https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-flash:countTokens?key=$GOOGLE_API_KEY \
+    -H 'Content-Type: application/json' \
+    -X POST \
+    -d '{
+      "contents": [{
+        "parts":[{
+          "text": "The quick brown fox jumps over the lazy dog."
+          }],
+        }],
+      }'
+# [END tokens_text_only]
+
+echo "[START tokens_chat]"
+# [START tokens_chat]
+curl https://generativelanguage.googleapis.com/v1beta/models/gemini-1.5-flash:countTokens?key=$GOOGLE_API_KEY \
+    -H 'Content-Type: application/json' \
+    -X POST \
+    -d '{
+      "contents": [
+        {"role": "user",
+        "parts": [{"text": "Hi, my name is Bob."}],
+        },
+        {"role": "model",
+         "parts":[{"text": "Hi Bob"}],
+        },
+      ],
+      }'
+# [END tokens_chat]