Can you please add the ability to retrieve and provide the code verifier

[Colin-b]

Right now, the auth code PKCE flow is using a code verifier generated when creating the authorization url.

This is fine, however I want to be able to use another OAuth2Session instance when requesting the token. To do so, I need to be able to get the code verifier that was generated, and provide it as a parameter to the fetch_token.

I can manage for now by accessing the private _code_verifier attribute, but I would like to rely on an interface that is not supposed to change without a new major release.

Thanks again