Secrets Provider - Implement Batch Retrieval API (Go SDK)

[sigalsax]

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As part of the Batch Milestone, we need to create a new endpoint to efficiently handle batch secret requests

This work can be split into 3:

1. Work in Conjur
2. Work in Secrets Provider
3. GO SDK that the Secrets Provider uses

DOD

• [ ] Create new API endpoint for batch retrieval in GO SDK
• [ ] Add unit tests