.Net: Optimize Pinecone GetBatchAsync to retrieve multiple entries with a single request

[atiq-bs23]

Motivation and Context

GetBatchAsync is sending n (equal to the number of keys) requests, where a single request could retrieve multiple entries at once.

Description

Now current implementation will execute a single request to retrieve multiple entries.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, integration test run is required.
• I didn't break anyone 😄