Providers like Okta and AWS Cognito expect that the PKCE challenge
uses base64 URL Encoding without any padding (base64.RawURLEncoding)
Additionally, Okta strictly adheres to section 4.2 of RFC 7636 and
requires that the unencoded key for the PKCE data is at least 43
characters in length.
These run against a stub OAuth server implementation, verifying that we
are able to run an end-to-end login transaction for both the authorization
code and the password grant types.
This includes adding support for authorization code grants to our stub
OAuth server implementation; it previously supported only the password
grant type.