Keycloak: Support username/password authentication

[ingokuba]

The Keycloak Terraform provider not only supports client-credentials grant, but also password grant type: https://registry.terraform.io/providers/keycloak/keycloak/latest/docs#example-usage-password-grant

provider "keycloak" {
    client_id     = "admin-cli"
    username      = "keycloak"
    password      = "password"
    url           = "http://localhost:8080"
}

It would be very helpful if this is provided by Terraformer aswell 🙏

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[ingokuba]

managed to build it locally - testing in progress