docs: Added FastAPI example. (#77)

* Added FastAPI example.

* Add required env vars to fastapi readme

---------

Co-authored-by: David Kegley <[email protected]>

examples/connect/fastapi/README.md

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+# FastAPI Example
+
+## Start the app locally
+
+```bash
+# install uvicorn
+pip install "uvicorn[standard]"
+
+# start the app
+export DATABRICKS_HOST="<databricks-sql-warehouse-server-hostname>"
+export DATABRICKS_PATH="<databricks-sql-warehouse-http-path>"
+uvicorn app:app --reload
+
+# send a request to the rest api
+curl -X 'GET' 'http://127.0.0.1:8000/fares'
+```
+
+## Deploy to Posit Connect
+
+Validate that `rsconnect-python` is installed:
+
+```bash
+rsconnect version
+```
+
+Or install it as documented in the [installation](https://docs.posit.co/rsconnect-python/#installation) section of the documentation.
+
+To publish, make sure `CONNECT_SERVER`, `CONNECT_API_KEY`, `DATABRICKS_HOST`, `DATABRICKS_PATH` have valid values. Then, on a terminal session, enter the following command:
+
+```bash
+rsconnect deploy fastapi . \
+  --server "${CONNECT_SERVER}" \
+  --api-key "${CONNECT_API_KEY}" \
+  --environment DATABRICKS_HOST \
+  --environment DATABRICKS_PATH
+```
+
+Note that the Databricks environment variables do not need to be resolved by the shell, so they do not include the `$` prefix.
+
+The Databricks environment variables only need to be set once, unless a change needs to be made. If the values have not changed, you don’t need to provide them again when you publish updates to the document.
+
+```
+rsconnect deploy fastapi . \
+  --server "${CONNECT_SERVER}" \
+  --api-key "${CONNECT_API_KEY}"
+```

examples/connect/fastapi/app.py

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+# -*- coding: utf-8 -*-
+# mypy: ignore-errors
+import os
+
+from posit.connect.external.databricks import viewer_credentials_provider
+
+from databricks import sql
+
+from typing import Annotated
+
+from fastapi import FastAPI, Header, Depends
+from fastapi.responses import JSONResponse
+
+DATABRICKS_HOST=os.getenv("DATABRICKS_HOST")
+DATABRICKS_HOST_URL = f"https://{DATABRICKS_HOST}"
+SQL_HTTP_PATH = os.getenv("DATABRICKS_PATH")
+
+rows = None
+app = FastAPI()
+
+async def get_token(
+    posit_connect_user_session_token: Annotated[str | None, Header()] = None
+) -> dict | None:
+    """
+    Get the token from the Posit-Connect-User-Session-Token header.
+    """
+    if posit_connect_user_session_token is None:
+        return None
+    return posit_connect_user_session_token
+
+@app.get("/fares")
+async def get_fares(session_token=Depends(get_token)) -> JSONResponse:
+    """
+    FastAPI example API that returns the first few rows from
+    a table hosted in Databricks.
+    """
+    global rows
+
+    credentials_provider = viewer_credentials_provider(user_session_token=session_token)
+
+    if rows is None:
+        query = "SELECT * FROM samples.nyctaxi.trips LIMIT 10;"
+
+        with sql.connect(server_hostname=DATABRICKS_HOST,
+                        http_path=SQL_HTTP_PATH,
+                        auth_type="databricks-oauth",
+                        credentials_provider=credentials_provider) as connection:
+            with connection.cursor() as cursor:
+                cursor.execute(query)
+                rows = cursor.fetchall()
+
+    return [row.asDict() for row in rows]

examples/connect/fastapi/requirements.txt

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+databricks-sql-connector==3.0.1
+databricks-sdk==0.20.0
+fastapi==0.110.0
+git+https://github.com/posit-dev/posit-sdk-py.git