Add simple Basic auth (#1203)

* Add simple Basic auth

To enable the basic authentication, one must set `server.auth.enabled`
to true.

The static string defined in `server.auth.secret` must be set in the
header `Authorization`.

The health check endpoint will always be accessible, no matter the API
auth configuration.

* Fix linting and type check

* Fighting with mypy being too restrictive

Had to disable mypy in the `auth` as we are not using the same signature
for the authenticated method.

mypy was complaining that the signatures of `authenticated` must be
identical, no matter in which logical branch we are.
Given that fastapi is accomodating itself of method signatures (it will
inject the dependencies in the method call), this warning of mypy is
actually preventing us to do something legit.

mypy doc: https://mypy.readthedocs.io/en/stable/common_issues.html

* Write tests to verify that the simple auth is working

private_gpt/server/completions/completions_router.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, Depends
 from pydantic import BaseModel
 from starlette.responses import StreamingResponse
 
@@ -8,8 +8,9 @@ from private_gpt.open_ai.openai_models import (
     OpenAIMessage,
 )
 from private_gpt.server.chat.chat_router import ChatBody, chat_completion
+from private_gpt.server.utils.auth import authenticated
 
-completions_router = APIRouter(prefix="/v1")
+completions_router = APIRouter(prefix="/v1", dependencies=[Depends(authenticated)])
 
 
 class CompletionsBody(BaseModel):