[terraform-ibm-rag-sample-da] Reduce number of inputs by asking for CRN instead

[ocofaigh]

We could reduce the number of inputs in the DA by asking for CRNs and parsing info from it (e.g. guid and region) instead of having two separate inputs asking for guid and region.
This approach would be consistent with other DAs.

For example:

• watson_machine_learning_instance_crn, watson_machine_learning_instance_guid and watson_machine_learning_instance_resource_name. The GUID can be parsed from the CRN, and the data lookup can be done on the GUID to get the resource name
• secrets_manager_guid and secrets_manager_region can be changed to 1 input: secrets_manager_crn

There are probably more examples like these.

Use the CRN parser to parse the info: https://github.com/terraform-ibm-modules/terraform-ibm-common-utilities/tree/main/modules/crn-parser