.Net: Adding a multi-store vector search sample.

[westey-m]

Motivation and Context

One of the main features of the vector store abstractions is to be able to easily swap out one store for another, so showing an example where we are loading data into a store and doing vector searches with the same code, and only the top level registration differs for different vector stores.

Description

• Adding a multi-store vector search example including Azure AI Search, Qdrant, Redis and Volatile
• Renaming simple search example to match naming, and make small refactoring.

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 I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

The class name with _ looks odd to me. We use the _ in some of the classes that are tests for readability bit in this case it's a utility class.