[terraform-ibm-code-engine] Update DAs to only support using existing resource group

[ocofaigh]

• As per our most recent discussions, we should no longer support creating a resource group in the DA, but instead we should require an existing resource group name (using input existing_resource_group_name).
  • Change is applicable for both the project and apps variations
  • We can still use our terraform-ibm-resource-group module in the code to return the resource group ID.
  • We should use the catalog resource group picker in the ibm_catalog.json for this input:

      {
          "key": "existing_resource_group_name",
          "custom_config": {
              "type": "resource_group",
              "grouping": "deployment",
              "original_grouping": "deployment",
              "config_constraints": {
                  "identifier": "rg_name"
              }
          }
      }
    

NOTE: This will be a breaking change and should require a major version update if the DA is already published in community