Enable OAuth Authorization Code Flow with PKCE in Swagger UI

[ellieayla]

Support for retrieving an authorization token via OAuth Implicit Flow in Swagger UI was added in #585.

From https://oauth.net/2/grant-types/implicit/

The Implicit grant type is a simplified flow that can be used by public clients, where the access token is returned immediately without an extra authorization code exchange step.
It is generally not recommended to use the implicit flow (and some servers prohibit this flow entirely). In the time since the spec was originally written, the industry best practice has changed to recommend that public clients should use the authorization code flow with the PKCE extension instead.

When swagger-api/swagger-ui#5348 is implemented, we should add support for Authorization Code Flow with PKCE (or verify it just works).

[TomaszLloyd]

I too would love to see this resolved