Enable client_secret_basic authentication on requestClientCredentialsToken()

[Magentron]

In order to be able to use the demo client credentials of Signicat to retrieve the client credentials token (a.k.a. access token) client secret basic authentication is required, because otherwise the error "Your client is configured to authenticate using CLIENT_SECRET_BASIC" is returned.

See:

• https://developer.signicat.com/enterprise/docs/identification/guides/#oidc-token
• https://developer.signicat.com/enterprise/docs/demo-service/#oidc-considerations

[Magentron]

@jumbojett Is there anything missing here for the corresponding PR to be accepted/merged?
We have been using this in production for about 1.5 years without issue.