Support PKCE for OAuth 2.0 in our OAuthBackend class

Beginning working on a backend for Airtable, I faced an issue that our OAuthBackend doesn't support the authorization parameter rules, which are mandatory for Airtable.

It turned out those rules are a part of the PKCE extension of OAuth. And it looks like those rules might become a part of the backends we might decide to support in the future. So it would be nice if we could support the PKCE extension of OAuth and allow backends to opt it in.