templates: Add rag lantern template (#16523)

Replace this entire comment with:
  - **Description:** Added a template for lantern rag usage.

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

templates/rag-lantern/README.md

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+
+# rag_lantern
+
+This template performs RAG with Lantern.
+
+[Lantern](https://lantern.dev) is an open-source vector database built on top of [PostgreSQL](https://en.wikipedia.org/wiki/PostgreSQL). It enables vector search and embedding generation inside your database.
+
+## Environment Setup
+
+Set the `OPENAI_API_KEY` environment variable to access the OpenAI models.
+
+To get your `OPENAI_API_KEY`, navigate to [API keys](https://platform.openai.com/account/api-keys) on your OpenAI account and create a new secret key.
+
+To find your `LANTERN_URL` and `LANTERN_SERVICE_KEY`, head to your Lantern project's [API settings](https://lantern.dev/dashboard/project/_/settings/api). 
+
+- `LANTERN_URL` corresponds to the Project URL
+- `LANTERN_SERVICE_KEY` corresponds to the `service_role` API key
+
+
+```shell
+export LANTERN_URL=
+export LANTERN_SERVICE_KEY=
+export OPENAI_API_KEY=
+```
+
+## Setup Lantern Database
+
+Use these steps to setup your Lantern database if you haven't already.
+
+1. Head to [https://lantern.dev](https://lantern.dev) to create your Lantern database.
+2. In your favorite SQL client, jump to the SQL editor and run the following script to setup your database as a vector store:
+
+   ```sql
+   -- Create a table to store your documents
+   create table
+     documents (
+       id uuid primary key,
+       content text, -- corresponds to Document.pageContent
+       metadata jsonb, -- corresponds to Document.metadata
+       embedding REAL[1536] -- 1536 works for OpenAI embeddings, change as needed
+     );
+
+   -- Create a function to search for documents
+   create function match_documents (
+     query_embedding REAL[1536],
+     filter jsonb default '{}'
+   ) returns table (
+     id uuid,
+     content text,
+     metadata jsonb,
+     similarity float
+   ) language plpgsql as $$
+   #variable_conflict use_column
+   begin
+     return query
+     select
+       id,
+       content,
+       metadata,
+       1 - (documents.embedding <=> query_embedding) as similarity
+     from documents
+     where metadata @> filter
+     order by documents.embedding <=> query_embedding;
+   end;
+   $$;
+   ```
+
+## Setup Environment Variables
+
+Since we are using [`Lantern`](https://python.langchain.com/docs/integrations/vectorstores/lantern) and [`OpenAIEmbeddings`](https://python.langchain.com/docs/integrations/text_embedding/openai), we need to load their API keys.
+
+## Usage
+
+First, install the LangChain CLI:
+
+```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 rag-lantern
+```
+
+If you want to add this to an existing project, you can just run:
+
+```shell
+langchain app add rag-lantern
+```
+
+And add the following code to your `server.py` file:
+
+```python
+from rag_lantern.chain import chain as rag_lantern_chain
+
+add_routes(app, rag_lantern_chain, path="/rag-lantern")
+```
+
+(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/rag-lantern/playground](http://127.0.0.1:8000/rag-lantern/playground)  
+
+We can access the template from code with:
+
+```python
+from langserve.client import RemoteRunnable
+
+runnable = RemoteRunnable("http://localhost:8000/rag-lantern")
+```