Add support to use s2s auth policy for IAM credentials

[ocofaigh]

In IBM-Cloud/terraform-provider-ibm#5678 (which was release in 1.7.0) support was added to use an s2s auth policy instead of an apikey for IAM credentials engine. We should expose that and make it the default in our module now (and any consuming examples / modules / DAs).

Ensure to bump the required provider version to >=1.70.0, <2.0.0

[alex-reiff]

They added support, but they didn't make the inputs for API key configuration optional. So right now if you enable s2s auth, it creates both configurations.

Issue opened with provider: IBM-Cloud/terraform-provider-ibm#5800