Update how batch secrets are retrieved

[telday]

Is your feature request related to a problem? Please describe.

A new feature was added to Conjur enabling a user to request secrets from the batch endpoint encoded in base64. This allows a user to retrieve binary secrets from the endpoint without Conjur throwing errors. The client should be updated to use this new feature to ensure there are no errors with secret retrieval.

Describe the solution you would like

Client now performs all batch secret requests with the Accept: base64 header specified.

Additional context

Here is the original issue referencing the use of secrets provider for k8s.