Add WithClientCredentialsAndTokenURL option about go-auth0 HOT 2 OPEN

Hey @medev21, thanks for the issue!

I'd like to try and understand why your domain and token URL are different as within the context of this SDK I believe it should always be on the same domain (whether that is the a custom domain or the default Auth0 tenant domain). What is the error you're receiving? Are you able to provide an example or some extra information about your setup?

if you're using a custom domain, calling WithClientCredentialsAndAudience with the audience value being your Management API Identifier (e.g. https://myaccount.auth0.com/api/v2/) should ensure the token is valid and can perform management actions.

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Hi @ewanharris! Sorry for the slow reply.

Unfortunately, I don't have context as to why the domain and the toke URL are different, that's how it was setup at my company and I don't have access to their Auth0 configuration.

The error I get when I use WithClient option
oauth2: Transport's Source is nil

As I mentioned above, this option only updates the m.http, so the m.tokenSource remains empty; from what I've seen from the SDK code for this option.

We have tried to use WithClientCredentialsAndAudience, but we get the same error. In the following code from internal/client:

https://github.com/auth0/go-auth0/blob/main/internal/client/client.go#L223

func OAuth2ClientCredentialsAndAudience(
	ctx context.Context,
	uri,
	clientID,
	clientSecret,
	audience string,
) oauth2.TokenSource {
	cfg := &clientcredentials.Config{
		ClientID:     clientID,
		ClientSecret: clientSecret,
		TokenURL:     uri + "/oauth/token",
		EndpointParams: url.Values{
			"audience": []string{audience},
		},
	}

	return cfg.TokenSource(ctx)
}

The uri is the domain, so there will be a mismatch since the token url and domain are different due to our unfortunate, I dare say questionable, setup. Therefore leading to an error.

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