Add Gmail Agent Example (#14567)

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

templates/openai-functions-agent-gmail/README.md

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+# OpenAI Functions Agent - Gmail
+
+This template implements a simple agent using OpenAI function calling imports directly from [langchain-core](https://pypi.org/project/langchain-core/) and [`langchain-community`](https://pypi.org/project/langchain-community/).
+
+This template creates an agent that uses OpenAI function calling to communicate its decisions on what actions to take. 
+
+This example creates an agent that can optionally look up information on the internet using Tavily's search engine.
+
+## Environment Setup
+
+The following environment variables need to be set:
+
+Set the `OPENAI_API_KEY` environment variable to access the OpenAI models.
+
+Create a [`credentials.json`](https://developers.google.com/gmail/api/quickstart/python#authorize_credentials_for_a_desktop_application) file containing your OAuth client ID from Gmail. To customize authentication, see the [Customize Auth](#customize-auth) section below.
+
+_*Note:* The first time you run this app, it will force you to go through a user authentication flow._
+
+## Usage
+
+To use this package, you should first have the LangChain CLI installed:
+
+```shell
+pip install -U langchain-cli
+```
+
+To create a new LangChain project and install this as the only package, you can do:
+
+```shell
+langchain app new my-app --package openai-functions-agent-gmail
+```
+
+If you want to add this to an existing project, you can just run:
+
+```shell
+langchain app add openai-functions-agent-gmail
+```
+
+And add the following code to your `server.py` file:
+```python
+from openai_functions_agent import agent_executor as openai_functions_agent_chain
+
+add_routes(app, openai_functions_agent_chain, path="/openai-functions-agent-gmail")
+```
+
+(Optional) Let's now configure LangSmith. 
+LangSmith will help us trace, monitor and debug LangChain applications. 
+LangSmith is currently in private beta, you can sign up [here](https://smith.langchain.com/). 
+If you don't have access, you can skip this section
+
+```shell
+export LANGCHAIN_TRACING_V2=true
+export LANGCHAIN_API_KEY=<your-api-key>
+export LANGCHAIN_PROJECT=<your-project>  # if not specified, defaults to "default"
+```
+
+If you are inside this directory, then you can spin up a LangServe instance directly by:
+
+```shell
+langchain serve
+```
+
+This will start the FastAPI app with a server is running locally at 
+[http://localhost:8000](http://localhost:8000)
+
+We can see all templates at [http://127.0.0.1:8000/docs](http://127.0.0.1:8000/docs)
+We can access the playground at [http://127.0.0.1:8000/openai-functions-agent-gmail/playground](http://127.0.0.1:8000/openai-functions-agent/playground)  
+
+We can access the template from code with:
+
+```python
+from langserve.client import RemoteRunnable
+
+runnable = RemoteRunnable("http://localhost:8000/openai-functions-agent-gmail")
+```
+
+## Customize Auth
+
+```
+from langchain.tools.gmail.utils import build_resource_service, get_gmail_credentials
+
+# Can review scopes here https://developers.google.com/gmail/api/auth/scopes
+# For instance, readonly scope is 'https://www.googleapis.com/auth/gmail.readonly'
+credentials = get_gmail_credentials(
+    token_file="token.json",
+    scopes=["https://mail.google.com/"],
+    client_secrets_file="credentials.json",
+)
+api_resource = build_resource_service(credentials=credentials)
+toolkit = GmailToolkit(api_resource=api_resource)
+```