Add support for custom domains

[msmolka]

Because previous task is closed, I'm asking again to add support for this.
We are validating issuer. Without this, this is tool is pretty useless

[larryaubstore]

It was possible until recently to manually change the authorize link for the custom domain in the debugger, but now I only see a GET https://{tenant}.wt.{domain}.auth0.com /auth0-authentication-api-debugger/pkce
and it's impossible to change anything manually.