Client: Include a buffer to the validity check.

[nam3less]

Describe the feature

I would like to propose a new kwarg for Clients which would define a "buffer" for the token validity check.
This option would let the client consider a token expired x seconds earlier then it does.
With a default value of 0 the change should be backwards compatible

Additional context

The feature would apply to the OAuth2 Client.

The highest difference could also be capped at a percentage of the tokens lifetime to prevent the token being considered expired
when it is newly received.

Background

Today I ran into an error where an API I used responded with 401 Not Authorized despite using the auto refresh feature of oauthlib. After investigating I think the problem was that the access_token was considered valid (not expired) when the
request was sent by the client but was expired when it reached the server.

Therefore I'm proposing this feature.

[JonathanHuot]

Hi, that's can be useful. Just by curiosity, have you checked if you can override a function instead of changing the kwarg?