.Net: Optimize DuckDb GetBatchAsync to retrieve multiple entries with a single query

[atiq-bs23]

Motivation and Context

GetBatchAsync is executing n (equal to the number of keys) queries, where a single query could retrieve multiple entries at once.

Description

Now current implementation will execute a single query 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, and existing test case can test this change.
• I didn't break anyone 😄