added history and support for system_message as param (#14824)

- **Description:** added support for chat_history for Google
GenerativeAI (to actually use the `chat` API) plus since Gemini
currently doesn't have a support for SystemMessage, added support for it
only if a user provides additional `convert_system_message_to_human`
flag during model initialization (in this case, SystemMessage would be
prepanded to the first HumanMessage)
  - **Issue:** #14710 
  - **Dependencies:** any dependencies required for this change,
- **Tag maintainer:** for a quicker response, tag the relevant
maintainer (see below),
  - **Twitter handle:** lkuligin

---------

Co-authored-by: William FH <13333726+hinthornw@users.noreply.github.com>

docs/docs/integrations/chat/google_generative_ai.ipynb

@@ -136,6 +136,32 @@
     "print(result.content)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "9e55d043-bb2f-44e3-9134-c39a1abe3a9e",
+   "metadata": {},
+   "source": [
+    "Gemini doesn't support `SystemMessage` at the moment, but it can be added to the first human message in the row. If you want such behavior, just set the `convert_system_message_to_human` to True:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "7a64b523-9710-4d15-9944-1e3cc567a52b",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.schema.messages import HumanMessage, SystemMessage\n",
+    "\n",
+    "model = ChatGoogleGenerativeAI(model=\"gemini-pro\", convert_system_message_to_human=True)\n",
+    "model(\n",
+    "    [\n",
+    "        SystemMessage(content=\"Answer only yes or no.\"),\n",
+    "        HumanMessage(content=\"Is apple a fruit?\"),\n",
+    "    ]\n",
+    ")"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "40773fac-b24d-476d-91c8-2da8fed99b53",