[terraform-ibm-rag-sample-da] Use prefix in secret names to prevent name clash

[ocofaigh]

Because secret names are hard coded in the DA, they can clash with other secrets. EG:

Error: CreateSecretWithContext failed A secret with the same name already exists: signing-key [secrets-manager.05047E]

Error: CreateSecretWithContext failed A secret with the same name already exists: ibmcloud-api-key

To prevent this, I think the DA should use the prefix in the secret name