extra_config is empty on keycloak_openid_client data source

[kherock]

When resolving a Keycloak client's details using data.keycloak_openid_client, the extra_config object computes to an empty object. It seems like there's some code in place that prevents this value from being computed properly:

https://github.com/mrparkers/terraform-provider-keycloak/blob/bfee4d7cd1591b015f4db84cce12bea36b6e871f/provider/extra_config.go#L47-L50

I presume this is to avoid interfering with existing attributes that are managed outside of Terraform. I think we need to disable this condition when the provider is computing the value for a data source.