[EN DA] Add support to store service credentials in secrets manager

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1. service_credential_names DA variable is a complex object type, and will be hard for consumers in Projects to know what format should be used for the values. I suggest that we add some supporting documentation for it, the same way we did here. Then we can update the variable description to point to the supporting doc
2. Once feat: add support for creating multiple secrets/secret_groups terraform-ibm-secrets-manager#157 is merged we should be able to call that module in the DA to add service credentials to a secrets manager secret

[ocofaigh]

#266